Keep Tests Honest in Multi-Window Claude Code Runs

Combine Anthropic's multi-context test integrity rule (tests must not be deleted or modified to pass builds) with single-test execution commands in CLAUDE.md.

## Testing Rules
- Execute single test files during development via: pnpm test:unit <file_path>
- Never delete, comment out, or modify existing assertions in tests.json or *.spec.ts files just to make a build pass.