Clarify behavioral test scope and stabilize Brooks import test - #299

Summary

• clarify that the repository behavioral principles also apply to services, workers, CLIs, and libraries while keeping userscripts-specific tooling local
• require an executable Given/When/Then acceptance scenario before production behavior changes
• document that review owns scenario meaning and test-first ordering while lint enforces only mechanical structure
• set explicit Brooks import fixture mtimes so the test models browser download order independently of filesystem timestamp coalescing

Validation

• targeted Brooks import test repeated 20/20 — passed
• npm run lint:tests — passed
• npm test — 2087 passed
• npm run test:affected -- --base d89445d537ffe230ff9ca2c79ccb9e1a68f5d531 — 1174 Node tests and 372 Playwright tests passed
• git diff --check — passed
• no userscript source or generated artifact changed; no version bump required