feat: add commit message normalizer

[marcus]

Summary

• add a reusable commit-message normalizer in internal/git with broader type support, td issue suffix handling, body/trailer preservation, and merge/autosquash exemptions
• add td commit-message / td commit-msg plus tracked commit-msg hook installation that works from linked worktrees
• update repo and website docs, and add regression coverage for normalization and hook installation

Verification

• go test ./internal/git ./cmd
• go test ./...
• manual smoke test in a temp repo covering git commit -m, editor-based git commit, and a no-focus docs changelog commit