chore: add commit message normalizer

[marcus]

Summary

• add td commit-message / td commit-msg normalization for conventional commit subjects
• install commit-msg alongside pre-commit, preserving commit bodies and trailers
• fix linked-worktree hook installation so make install-hooks points at the active checkout

Testing

• go build ./...
• go test ./internal/git ./cmd
• go test ./...
• manual smoke test in a temp repo + linked worktree: install hooks, focus an issue, verify a loose subject is rewritten while trailers are preserved, and confirm merge/revert/autosquash subjects stay unchanged

Supersedes #141 with the linked-worktree hook fix from review.