chore: add commit message normalizer

[marcus]

Summary

• add a repo-owned commit-msg hook that normalizes Conventional Commit subjects and preserves bodies/trailers
• install the commit-msg hook alongside pre-commit via make install-hooks
• document the commit format and update release automation/examples

Verification

• scripts/test-commit-msg-hook.sh
• make -n install-hooks
• go test ./...

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