Extension system: Add extension filtering for ext-specific commands #1848
Description
Summary
Only register extension-specific commands if the target extension is actually installed, preventing orphaned commands.
Context
Presets intelligently filter command overrides (presets.py:391-402) - only register extension-specific commands (pattern: speckit.<ext-id>.<cmd-name>) if the extension is actually installed.
Extensions don't implement this filtering - if an extension provides commands, they're all registered regardless of dependencies.
Requirements
- Detect extension-specific command patterns in manifest
- Check if target extension is installed before registering
- Skip orphaned commands with warning
- Re-evaluate on extension install/remove
Related
- Part of extension system parity with preset system