Fix commit message breaking marker preservation

[marcus]

Summary

• add td-native commit message normalization plus commit-msg hook/docs from the reviewed v21 task branch
• preserve conventional-commit breaking markers like feat!: and fix(scope)!: during subject normalization and file rewrites
• keep no-issue docs/test/chore/ci commits, hook installation for linked worktrees, and body/trailer preservation intact

Verification

• go test ./internal/git ./cmd
• go test ./...
• manual smoke test in a disposable linked worktree: make install-hooks installed stable symlinks, git commit rewrote only the subject line, preserved !, and left the body/trailer untouched