Add commit message normalizer

[marcus]

Summary

• add a repo-level commit-msg hook that normalizes safe subject-line inconsistencies and rejects malformed commit messages
• install the new hook from make install-hooks and update contributor docs/prompting to use one canonical format
• align release automation with the same type: summary subject style

Verification

• make install-hooks
• bash -n scripts/commit-msg.sh && bash -n scripts/pre-commit.sh
• throwaway git repo commit flow: valid subject accepted, fixable subject rewritten, malformed subject rejected, trailers preserved
• go build ./...

Test Status

• make test still fails in internal/features on this branch and on clean main with existing failures in TestKnownFeatureDefaults, TestIsEnabledForProcess_EnableDisableLists, and TestIsEnabled_ProjectConfigAndEnvPrecedence