Add pre-commit hook and CLAUDE.md

[jpr5]

Summary

• Add husky + lint-staged pre-commit hook that runs prettier and eslint on staged files automatically
• Add CLAUDE.md documenting check workflow and project conventions
• Prevents CI failures from formatting/lint issues by catching them before commit

Test plan

• pnpm run format:check passes
• pnpm run lint passes
• pnpm test passes (259 tests)
• Pre-commit hook tested — lint-staged runs on commit