chore(repo): normalize commit messages

[marcus]

Summary

• standardize future commit subjects with a commit-msg hook and updated README guidance
• accept Conventional Commits breaking-change syntax and install hooks safely in linked worktrees
• validate PR titles in CI using the base branch validator so squash merges on main stay protected

Testing

• bash ./scripts/commit-msg.sh --title "feat!: drop legacy API"
• bash ./scripts/commit-msg.sh --title "feat(api)!: drop legacy API"
• bash ./scripts/commit-msg.sh --title "bad message"
• make install-hooks
• make -C -f /Users/marcus/code/nightshift/Makefile install-hooks
• go test ./...

Supersedes #112.