feat(presets): Add composition strategies (prepend, append, wrap) for templates, commands, and scripts

- Add strategy field validation in PresetManifest._validate()
- Add VALID_PRESET_STRATEGIES and VALID_SCRIPT_STRATEGIES constants
- Add PresetResolver.resolve_content() for composed content resolution
- Add PresetResolver._collect_all_layers() for full priority stack
- Update _register_commands() to compose before writing
- Update CLI preset resolve command to show composition chain
- Add resolve_template_content() to bash common.sh
- Add Resolve-TemplateContent to PowerShell common.ps1
- Update scaffold preset.yml with strategy documentation
- Update presets/README.md and ARCHITECTURE.md
- Add 26 unit tests covering validation, composition, and chaining

Agent-Logs-Url: https://github.com/github/spec-kit/sessions/c285c51b-f00b-480a-9eb2-ae70e3cbcb72

Co-authored-by: mnriem <15701806+mnriem@users.noreply.github.com>

Author: Copilot

presets/ARCHITECTURE.md

@@ -41,6 +41,24 @@ The resolution is implemented three times to ensure consistency:
 - **Bash**: `resolve_template()` in `scripts/bash/common.sh`
 - **PowerShell**: `Resolve-Template` in `scripts/powershell/common.ps1`
 
+### Composition Strategies
+
+Templates, commands, and scripts support a `strategy` field that controls how a preset's content is combined with lower-priority content instead of fully replacing it:
+
+| Strategy | Description | Templates | Commands | Scripts |
+|----------|-------------|-----------|----------|---------|
+| `replace` (default) | Fully replaces lower-priority content | ✓ | ✓ | ✓ |
+| `prepend` | Places content before lower-priority content (separated by a blank line) | ✓ | ✓ | — |
+| `append` | Places content after lower-priority content (separated by a blank line) | ✓ | ✓ | — |
+| `wrap` | Content contains `{CORE_TEMPLATE}` (templates/commands) or `$CORE_SCRIPT` (scripts) placeholder replaced with lower-priority content | ✓ | ✓ | ✓ |
+
+Composition is recursive — multiple composing presets chain. The `PresetResolver.resolve_content()` method walks the full priority stack bottom-up and applies each layer's strategy.
+
+Content resolution functions for composition:
+- **Python**: `PresetResolver.resolve_content()` in `src/specify_cli/presets.py`
+- **Bash**: `resolve_template_content()` in `scripts/bash/common.sh`
+- **PowerShell**: `Resolve-TemplateContent` in `scripts/powershell/common.ps1`
+
 ## Command Registration
 
 When a preset is installed with `type: "command"` entries, the `PresetManager` registers them into all detected agent directories using the shared `CommandRegistrar` from `src/specify_cli/agents.py`.

presets/README.md

@@ -61,7 +61,37 @@ specify preset add healthcare-compliance --priority 5  # overrides enterprise-sa
 specify preset add pm-workflow --priority 1            # overrides everything
 ```
 
-Presets **override**, they don't merge. If two presets both provide `spec-template`, the one with the lowest priority number wins entirely.
+Presets **override by default**, they don't merge. If two presets both provide `spec-template` with the default `replace` strategy, the one with the lowest priority number wins entirely. However, presets can use **composition strategies** to augment rather than replace content.
+
+### Composition Strategies
+
+Presets can declare a `strategy` per template to control how content is combined:
+
+```yaml
+provides:
+  templates:
+    - type: "template"
+      name: "spec-template"
+      file: "templates/spec-addendum.md"
+      strategy: "append"        # adds content after the core template
+```
+
+| Strategy | Description |
+|----------|-------------|
+| `replace` (default) | Fully replaces the lower-priority template |
+| `prepend` | Places content **before** the resolved lower-priority template, separated by a blank line |
+| `append` | Places content **after** the resolved lower-priority template, separated by a blank line |
+| `wrap` | Content contains `{CORE_TEMPLATE}` placeholder (or `$CORE_SCRIPT` for scripts) replaced with the lower-priority content |
+
+**Supported combinations:**
+
+| Type | `replace` | `prepend` | `append` | `wrap` |
+|------|-----------|-----------|----------|--------|
+| **template** | ✓ (default) | ✓ | ✓ | ✓ |
+| **command** | ✓ (default) | ✓ | ✓ | ✓ |
+| **script** | ✓ (default) | — | — | ✓ |
+
+Multiple composing presets chain recursively. For example, a security preset with `prepend` and a compliance preset with `append` will produce: security header + core content + compliance footer.
 
 ## Catalog Management
 
@@ -108,13 +138,5 @@ See [scaffold/](scaffold/) for a scaffold you can copy to create your own preset
 
 The following enhancements are under consideration for future releases:
 
-- **Composition strategies** — Allow presets to declare a `strategy` per template instead of the default `replace`:
-
-  | Type | `replace` | `prepend` | `append` | `wrap` |
-  |------|-----------|-----------|----------|--------|
-  | **template** | ✓ (default) | ✓ | ✓ | ✓ |
-  | **command** | ✓ (default) | ✓ | ✓ | ✓ |
-  | **script** | ✓ (default) | — | — | ✓ |
-
-  For artifacts and commands (which are LLM directives), `wrap` would inject preset content before and after the core template using a `{CORE_TEMPLATE}` placeholder. For scripts, `wrap` would run custom logic before/after the core script via a `$CORE_SCRIPT` variable.
-- **Script overrides** — Enable presets to provide alternative versions of core scripts (e.g. `create-new-feature.sh`) for workflow customization. A `strategy: "wrap"` option could allow presets to run custom logic before/after the core script without fully replacing it.
+- **Structural merge strategies** — Parsing Markdown sections for per-section granularity (e.g., "replace only ## Security").
+- **Conflict detection** — `specify preset lint` / `specify preset doctor` for detecting composition conflicts.

presets/scaffold/preset.yml

@@ -32,6 +32,14 @@ provides:
   templates:
     # CUSTOMIZE: Define your template overrides
     # Templates are document scaffolds (spec-template.md, plan-template.md, etc.)
+    #
+    # Strategy options (optional, defaults to "replace"):
+    #   replace  - Fully replaces the lower-priority template (default)
+    #   prepend  - Places this content BEFORE the lower-priority template
+    #   append   - Places this content AFTER the lower-priority template
+    #   wrap     - Uses {CORE_TEMPLATE} placeholder, replaced with lower-priority content
+    #
+    # Note: Scripts only support "replace" and "wrap" strategies.
     - type: "template"
       name: "spec-template"
       file: "templates/spec-template.md"
@@ -45,6 +53,22 @@ provides:
     #   description: "Custom plan template"
     #   replaces: "plan-template"
 
+    # COMPOSITION EXAMPLES:
+    # Append additional sections to an existing template:
+    # - type: "template"
+    #   name: "spec-template"
+    #   file: "templates/spec-addendum.md"
+    #   description: "Add compliance section to spec template"
+    #   strategy: "append"
+    #
+    # Wrap a command with preamble/sign-off:
+    # - type: "command"
+    #   name: "speckit.specify"
+    #   file: "commands/specify-wrapper.md"
+    #   description: "Wrap specify command with compliance checks"
+    #   strategy: "wrap"
+    #   # In the wrapper file, use {CORE_TEMPLATE} where the original content goes
+
     # OVERRIDE EXTENSION TEMPLATES:
     # Presets sit above extensions in the resolution stack, so you can
     # override templates provided by any installed extension.

scripts/bash/common.sh

@@ -373,3 +373,146 @@ except Exception:
     return 1
 }
 
+# Resolve a template name to composed content using composition strategies.
+# Reads strategy metadata from preset manifests and composes content
+# from multiple layers using prepend, append, or wrap strategies.
+#
+# Usage: CONTENT=$(resolve_template_content "template-name" "$REPO_ROOT")
+# Returns composed content string on stdout; exit code 1 if not found.
+resolve_template_content() {
+    local template_name="$1"
+    local repo_root="$2"
+    local base="$repo_root/.specify/templates"
+
+    # Collect all layers (highest priority first)
+    local -a layer_paths=()
+    local -a layer_strategies=()
+
+    # Priority 1: Project overrides (always "replace")
+    local override="$base/overrides/${template_name}.md"
+    if [ -f "$override" ]; then
+        layer_paths+=("$override")
+        layer_strategies+=("replace")
+    fi
+
+    # Priority 2: Installed presets (sorted by priority from .registry)
+    local presets_dir="$repo_root/.specify/presets"
+    if [ -d "$presets_dir" ]; then
+        local registry_file="$presets_dir/.registry"
+        local sorted_presets=""
+        if [ -f "$registry_file" ] && command -v python3 >/dev/null 2>&1; then
+            if sorted_presets=$(SPECKIT_REGISTRY="$registry_file" python3 -c "
+import json, sys, os
+try:
+    with open(os.environ['SPECKIT_REGISTRY']) as f:
+        data = json.load(f)
+    presets = data.get('presets', {})
+    for pid, meta in sorted(presets.items(), key=lambda x: x[1].get('priority', 10)):
+        print(pid)
+except Exception:
+    sys.exit(1)
+" 2>/dev/null); then
+                if [ -n "$sorted_presets" ]; then
+                    while IFS= read -r preset_id; do
+                        local candidate="$presets_dir/$preset_id/templates/${template_name}.md"
+                        if [ -f "$candidate" ]; then
+                            # Read strategy from preset manifest
+                            local strategy="replace"
+                            local manifest="$presets_dir/$preset_id/preset.yml"
+                            if [ -f "$manifest" ] && command -v python3 >/dev/null 2>&1; then
+                                local s
+                                s=$(SPECKIT_MANIFEST="$manifest" SPECKIT_TMPL="$template_name" python3 -c "
+import yaml, sys, os
+try:
+    with open(os.environ['SPECKIT_MANIFEST']) as f:
+        data = yaml.safe_load(f)
+    for t in data.get('provides', {}).get('templates', []):
+        if t.get('name') == os.environ['SPECKIT_TMPL'] and t.get('type', 'template') == 'template':
+            print(t.get('strategy', 'replace'))
+            sys.exit(0)
+    print('replace')
+except Exception:
+    print('replace')
+" 2>/dev/null) && strategy="$s"
+                            fi
+                            layer_paths+=("$candidate")
+                            layer_strategies+=("$strategy")
+                        fi
+                    done <<< "$sorted_presets"
+                fi
+            fi
+        fi
+    fi
+
+    # Priority 3: Extension-provided templates (always "replace")
+    local ext_dir="$repo_root/.specify/extensions"
+    if [ -d "$ext_dir" ]; then
+        for ext in "$ext_dir"/*/; do
+            [ -d "$ext" ] || continue
+            case "$(basename "$ext")" in .*) continue;; esac
+            local candidate="$ext/templates/${template_name}.md"
+            if [ -f "$candidate" ]; then
+                layer_paths+=("$candidate")
+                layer_strategies+=("replace")
+            fi
+        done
+    fi
+
+    # Priority 4: Core templates (always "replace")
+    local core="$base/${template_name}.md"
+    if [ -f "$core" ]; then
+        layer_paths+=("$core")
+        layer_strategies+=("replace")
+    fi
+
+    local count=${#layer_paths[@]}
+    [ "$count" -eq 0 ] && return 1
+
+    # Check if any layer uses a non-replace strategy
+    local has_composition=false
+    for s in "${layer_strategies[@]}"; do
+        [ "$s" != "replace" ] && has_composition=true && break
+    done
+
+    if [ "$has_composition" = false ]; then
+        cat "${layer_paths[0]}"
+        return 0
+    fi
+
+    # Compose bottom-up: start from lowest priority
+    local content=""
+    local started=false
+    local i
+    for (( i=count-1; i>=0; i-- )); do
+        local path="${layer_paths[$i]}"
+        local strat="${layer_strategies[$i]}"
+        local layer_content
+        layer_content=$(cat "$path")
+
+        if [ "$started" = false ]; then
+            if [ "$strat" = "replace" ]; then
+                content="$layer_content"
+            fi
+            # Keep consuming replace layers from the bottom until we hit a non-replace
+            if [ "$strat" != "replace" ]; then
+                started=true
+                case "$strat" in
+                    prepend) content="${layer_content}\n\n${content}" ;;
+                    append)  content="${content}\n\n${layer_content}" ;;
+                    wrap)    content="${layer_content//\{CORE_TEMPLATE\}/$content}" ;;
+                esac
+            fi
+        else
+            case "$strat" in
+                replace) content="$layer_content" ;;
+                prepend) content="${layer_content}\n\n${content}" ;;
+                append)  content="${content}\n\n${layer_content}" ;;
+                wrap)    content="${layer_content//\{CORE_TEMPLATE\}/$content}" ;;
+            esac
+        fi
+    done
+
+    printf '%s' "$content"
+    return 0
+}
+