docs: testing policy in CONTRIBUTING + PR template with verification checklist

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>

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,30 @@
+## Summary
+
+<!-- What does this PR do, in one or two sentences? -->
+
+## Why / Motivation
+
+<!-- What problem does this solve? Why now, and why this approach?
+     Link the issue or context that prompted it. -->
+
+## Changes
+
+<!-- The notable changes, at a high level. Describe the outcome, not the diff. -->
+
+## Tradeoffs & risks
+
+<!-- What did you weigh? What could break, what's out of scope, and what
+     follow-ups (if any) does this leave behind? Note any security or
+     migration implications. -->
+
+## How verified
+
+<!-- How do you know this works? Tests added/run, manual steps, commands,
+     screenshots. Include the exact commands where relevant. -->
+
+## Checklist
+
+- [ ] Ran the relevant build/test command for the area changed.
+- [ ] Reviewed `git diff` and removed unrelated changes.
+- [ ] Updated docs / env examples if setup, config, or behavior changed.
+- [ ] No secrets, API keys, real documents, or `.env` files committed.