Enforce Conventional Commits format with pre-commit hook

[smoparth]

Problem

CONTRIBUTING.md requires Conventional Commit format (feat:, fix:, refactor:, etc.) but there is no tooling to enforce it. Non-conforming commit messages (e.g., "fixed the thing", "wip") can be pushed without any validation, making commit history inconsistent and harder to parse for changelog generation, semantic versioning, and AI-assisted development workflows.

The .pre-commit-config.yaml already has hooks for code quality (ruff, mypy) but nothing for commit message validation.

Context

• The project already uses .pre-commit-config.yaml with local hooks wired through hatch.
• CONTRIBUTING.md documents the expected commit format under "Commit Guidelines".
• AGENTS.md also references Conventional Commits as the required format for AI agents.
• The gap is enforcement — the convention is documented but not validated.

Definition of Done

• A commit-msg hook validates commit messages against the Conventional Commits spec
• Non-conventional messages are rejected locally with a clear error
• Standard types pass: feat:, fix:, refactor:, docs:, test:, chore:, ci:
• Scoped commits pass: feat(resolver):, fix(bootstrapper):
• Hook is installed via the existing hatch run lint:install-hooks workflow
• Existing pre-commit hooks (ruff, mypy) are unaffected
• CONTRIBUTING.md updated if install instructions need changes