Context

Rex's review of #17 flagged that the commit-format regex and the PR-title regex in `git-conventions.md` both reject `feat!: breaking change` (the Conventional Commits 1.0 breaking-change marker). This is consistent between the hook and the existing convention, so it's not a bug — but Conventional Commits compliance is a common ask and the exclamation-mark marker is how most teams indicate breaking changes.

Design question

Do we want to support the Conventional Commits 1.0 breaking-change marker across the two hooks + the rule file?

Pro: industry-standard notation, some tools (semantic-release, commitlint, changelog generators) expect it.

Con: adds complexity, the existing `BREAKING CHANGE:` footer convention already handles the semantic need, and we'd have to decide whether to hard-block on missing `BREAKING CHANGE:` footer when `!` is present (Conventional Commits spec says it's optional when `!` is used).

Acceptance Criteria (if we decide to support it)

• Extend the type regex to `^(feat|fix|...)(\([^)]+\))?!?:[[:space:]]+.+` in both:
  • `.claude/hooks/validate-commit-format.sh`
  • `.claude/hooks/validate-pr-create.sh` (the title-format check)
  • `.claude/rules/git-conventions.md` (the stated regex)
• Decision: when `!` is present, do we require a `BREAKING CHANGE:` footer in the commit body? Either:
  • (a) Yes (stricter) — add a verify-breaking-change-footer.sh hook that blocks commits with `!` but no footer.
  • (b) No (looser) — trust the `!` alone as the marker. Matches spec.
• Write a short AgDR recording the `!` support decision and the footer-requirement decision.
• Smoke test: `feat!: breaking subject` is now accepted; `feat(api)!: breaking API change` is accepted.

Non-goals

• Does NOT automate changelog generation from breaking-change markers.
• Does NOT integrate with semantic-release.

Links

• Root-cause: Rex's review of feat(#13): rule-audit hooks — AgDR-for-arch, design review, red-CI, commit-format #17 / nice-to-have fix(#99): post-merge cleanup from PR #1 Rex review #3
• Related: [Chore] audit CLAUDE.md and mechanize MUST rules into hooks #13 (closed)
• Conventional Commits spec: https://www.conventionalcommits.org/en/v1.0.0/