chore: normalize commit messages

[gregggreg]

Summary

• add a repository-managed Conventional Commit normalizer for future commits only; existing history is not rewritten
• normalize unambiguous type, scope, and whitespace while preserving bodies, Nightshift trailers, and commit-message file identity
• exempt Git-generated merge, revert, autosquash, stash, and combined-commit messages, including custom comment prefixes
• adopt .githooks without dropping the existing formatting, vet, and build checks
• add a conflict-safe, repository-local hook installer plus shell regression coverage and developer documentation

Iteration 3

• inspect every repository-local core.hooksPath value instead of only the last value
• reject ambiguous duplicate, conflicting, reordered, and empty additional hook-path values without modifying .git/config
• add regression coverage for both value orderings and the empty-value edge case

Verification

• make test-commit-normalizer (74 assertions)
• make test (74 shell assertions and all Go tests)
• sh -n scripts/normalize-commit-message.sh scripts/install-hooks.sh .githooks/commit-msg .githooks/pre-commit tests/commit-message-normalizer.sh
• end-to-end accepted and rejected Git commit coverage (included in the shell suite)
• git diff --check docs-backfill