fix: Address code review feedback - edge case in bash/ps1 composition, improve default strategy test

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

scripts/bash/common.sh

@@ -481,6 +481,7 @@ except Exception:
 
     # Compose bottom-up: start from lowest priority
     local content=""
+    local has_base=false
     local started=false
     local i
     for (( i=count-1; i>=0; i-- )); do
@@ -492,9 +493,12 @@ except Exception:
         if [ "$started" = false ]; then
             if [ "$strat" = "replace" ]; then
                 content="$layer_content"
+                has_base=true
             fi
             # Keep consuming replace layers from the bottom until we hit a non-replace
             if [ "$strat" != "replace" ]; then
+                # No base content to compose onto
+                [ "$has_base" = false ] && return 1
                 started=true
                 case "$strat" in
                     prepend) content="${layer_content}\n\n${content}" ;;

scripts/powershell/common.ps1

@@ -482,6 +482,8 @@ except Exception:
                 $content = $layerContent
             }
             if ($strat -ne 'replace') {
+                # No base content to compose onto
+                if ($null -eq $content) { return $null }
                 $started = $true
                 switch ($strat) {
                     'prepend' { $content = "$layerContent`n`n$content" }

tests/test_presets.py

@@ -3092,8 +3092,11 @@ def test_valid_wrap_strategy(self, temp_dir, valid_pack_data):
         assert manifest.templates[0]["strategy"] == "wrap"
 
     def test_default_strategy_is_replace(self, pack_dir):
-        """Test that omitting strategy defaults to replace."""
+        """Test that omitting strategy defaults to replace (key is absent)."""
         manifest = PresetManifest(pack_dir / "preset.yml")
+        # Strategy key should not be present in the manifest data
+        assert "strategy" not in manifest.templates[0]
+        # But consumers should treat missing strategy as "replace"
         assert manifest.templates[0].get("strategy", "replace") == "replace"
 
     def test_invalid_strategy_rejected(self, temp_dir, valid_pack_data):