Add documentation for contextual help dialog

Assisted-By: docker-agent

Author: masegraye

docs/HELP_DIALOG.md

@@ -0,0 +1,68 @@
+# Contextual Help Dialog
+
+## Overview
+
+The cagent TUI now includes a contextual help dialog that displays all currently active key bindings. This makes it easy to discover available keyboard shortcuts without leaving the application.
+
+## Usage
+
+Press **Ctrl+?** (Ctrl+Shift+/) to open the help dialog.
+
+The dialog will show all key bindings that are currently active based on:
+- Which panel is focused (content view vs editor)
+- Whether lean mode is enabled
+- Terminal capabilities (keyboard enhancements support)
+- Current UI state (history search active, etc.)
+
+## Features
+
+### Contextual Bindings
+The help dialog is **context-aware** - it shows only the key bindings that are valid for your current state:
+
+- **When editor is focused**: Shows editor-specific bindings (Ctrl+g for external editor, Ctrl+r for history search, etc.)
+- **When content is focused**: Shows content view bindings (navigation, inline editing, etc.)
+- **Always visible**: Global bindings like Ctrl+c (quit), Tab (switch focus), Ctrl+k (commands)
+
+### Automatic Categorization
+Key bindings are automatically grouped into three categories for easy scanning:
+
+1. **General** - Common keys like Esc, Enter, Tab
+2. **Control Key Shortcuts** - All Ctrl+ combinations
+3. **Other** - Any remaining bindings
+
+### Scrollable
+If there are more key bindings than fit on screen, the dialog is scrollable:
+- Use **↑/↓** arrow keys or **PgUp/PgDn** to scroll
+- Press **Esc** or **Enter** to close the dialog
+
+## Implementation Details
+
+The help dialog leverages the existing `Bindings()` method structure in the TUI, which already returns different key bindings based on context. This means:
+
+- The help is always accurate and up-to-date
+- No manual maintenance of help text needed
+- Automatically adapts to new features and key bindings
+
+### For Developers
+
+To add new key bindings to the help:
+
+1. Create your `key.Binding` with `.WithHelp()` to provide description
+2. Add it to the appropriate `Bindings()` method (app, component, or page level)
+3. It will automatically appear in the help dialog
+
+Example:
+```go
+newFeatureKey := key.NewBinding(
+    key.WithKeys("ctrl+f"),
+    key.WithHelp("Ctrl+f", "new feature"),
+)
+```
+
+The help dialog will pick this up automatically when it's included in any `Bindings()` return value.
+
+## Related Files
+
+- `pkg/tui/dialog/help.go` - Help dialog implementation
+- `pkg/tui/tui.go` - Key binding handler (Ctrl+? binding)
+- `pkg/tui/dialog/readonly_scroll_dialog.go` - Base dialog with scrolling support