feat: add commit message normalizer

[marcus]

Summary

• add td commit-message / td commit-msg, commit-message normalization helpers, and tracked commit-msg hook installation
• preserve commit bodies and trailers while normalizing only the subject line, with worktree-safe hook symlinks and updated docs
• leave Git-generated merge, revert, and autosquash subjects untouched so git revert and other special flows keep working

Testing

• go test ./internal/git ./cmd
• go test ./...
• manual temp-repo exercise of git commit -m, editor-based git commit, and git revert --no-edit with the installed hooks