Add check-commit command to gsd-tools that acts as a pre-commit guard.
When commit_docs is false, rejects commits that stage .planning/ files
with an actionable error message including the unstage command.
Recreated cleanly on current main — previous version carried stale
shared fixes that are now upstream.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>