Adds a Testing section documenting every suite the testing PR series
introduces (unit/integration via vitest, Playwright e2e, offline evals,
gated real-Supabase stack tests), the expectation that changes carry tests
at the lowest layer that catches the regression, and a PR template (ported
from the amal66 fork, Open-Legal-Products/mike#205) whose checklist asks how
the change was verified. Intended as the capstone of the series — the
commands it documents are introduced by the sibling test PRs.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
