Add commit message normalizer

[marcus]

Summary

• add td commit-message / td commit-msg to normalize loose subjects, preserve bodies and trailers, and skip git-generated merge/revert/autosquash subjects
• install a repo commit-msg hook alongside pre-commit, with worktree-safe hook symlinks
• update contributor and agent docs to make make install-hooks + td commit-message the canonical commit workflow

Testing

• go test ./internal/git ./cmd
• go test ./...
• manual smoke test in a temp checkout covering subject rewrite, body/trailer preservation, and fixup/revert/merge exemptions