chore: normalize commit message enforcement

[marcus]

Summary

• add a Conventional Commits validator for local commit messages
• install worktree-aware git hook wrappers for both pre-commit and commit-msg
• validate PR titles in CI using the base-branch validator with a bootstrap fallback

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 "fix(schedule.max_projects config value ignored by run command; budget calibration broken at day boundary #19): align budget day boundary"
• make install-hooks
• go test ./...