Add a shared linter (.tangled/hooks/commit-msg-lint.sh) enforcing the Go / Tailscale commit convention: a real pkg/path prefix, imperative lowercase summary, no trailing period, blank line before the body. It rejects Conventional Commit prefixes (fix:, feat:) and invented roots like workflows/ (the real path is .tangled/workflows). Wire it up two ways: a commit-lint CI workflow that lints the PR commit range (or the pushed tip), and a git commit-msg hook installable via .tangled/hooks/install.sh. jj users rely on CI or run the linter by hand. Document the convention and setup in CONTRIBUTING.md.