Extension system: Add extension filtering for ext-specific commands

[mbachorik]

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

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[github-actions]

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