docs: add pull request template

.github/pull_request_template.md

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+<!--
+Thanks for the PR! Please fill out the sections below so reviewers have the
+context they need. Delete sections that genuinely don't apply, but prefer
+filling them out — even a one-line answer beats a missing section.
+
+PR title should follow Conventional Commits, e.g.:
+  feat(web): add password reset flow
+  fix(api): handle null session in /me
+  chore: bump pnpm to 11.0.9
+-->
+
+## Summary
+
+<!-- What does this PR do, and why? Lead with the user-visible or behavioral change. -->
+
+## Linked issues
+
+<!-- e.g. Closes #123, Refs #456. Use "Closes" so the issue auto-closes on merge. -->
+
+## Affected area
+
+<!-- Mirror the issue-template dropdown. Tick all that apply. -->
+
+- [ ] apps/web
+- [ ] apps/api
+- [ ] packages/shared
+- [ ] packages/db
+- [ ] packages/auth
+- [ ] packages/api-client
+- [ ] packages/api-spec
+- [ ] tooling / CI
+- [ ] other / cross-cutting
+
+## Type of change
+
+- [ ] feat — new user-facing capability
+- [ ] fix — bug fix
+- [ ] chore — tooling, deps, or maintenance
+- [ ] refactor — no behavior change
+- [ ] docs — documentation only
+- [ ] test — tests only
+
+## How was this tested?
+
+<!--
+Describe what you ran locally and what you verified. Include relevant commands,
+screenshots, or recordings for UI changes.
+-->
+
+- [ ] `pnpm lint`
+- [ ] `pnpm lint:pinning` (if Docker tags or `package.json` deps changed)
+- [ ] `pnpm test`
+- [ ] `pnpm build`
+- [ ] Manual verification (describe below)
+
+## Schema / migration impact
+
+<!--
+Required if this PR touches `packages/db`, `packages/shared` schemas, or the
+OpenAPI spec. Otherwise write "none".
+-->
+
+- [ ] Prisma migration added / regenerated
+- [ ] `pnpm codegen:openapi` re-run and committed
+- [ ] No schema impact
+
+## Additional context
+
+<!-- Anything else reviewers should know: trade-offs, follow-ups, related PRs. -->