Add automated SDD documentation drift detection tooling:
- /spec-drift: full scan command for validating all ai-docs against source code
- /spec-drift-changed: incremental scan for changed files only
- Pre-commit hook to block commits when ai-docs drift is unverified

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

- Register hook in settings.json (was dead code without it)
- Fail-closed when python3 unavailable or JSON parse fails
- Use content-based hash (git diff --cached) instead of path-only hash
- Don't consume marker in PreToolUse — invalidated naturally by content changes
- Precise case pattern to avoid matching git commit-tree
- Add shasum/sha256sum portability fallback
- Fix "Task tool" → "Agent tool" references in command files
- Add agent timeout handling rule
- Add staged-only content note for verification marker
- Make widget path mappings dynamic instead of hardcoded
- Label inventory as example
- Document advisory-only nature of verification

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>