chore: add a pull request template (#3259)

* chore: add pull request template

* fix: address Copilot review on PR template

- Reword the issue-link comment (plain #123 links; Fixes/Closes only auto-closes)
- Remove the standalone '-' bullets under Bug fix verification / Validation
- Align Validation commands with CI (frontend format + build with BETTER_AUTH_SECRET)

.github/pull_request_template.md

@@ -0,0 +1,61 @@
+<!-- Reference a related issue with #123. Use Fixes / Closes / Resolves to
+     auto-close it on merge. Delete this line if the PR doesn't reference an issue. -->
+Fixes #
+
+## Why
+
+<!-- Why are you opening this PR? Cover two things:
+       - The trigger — what made you write this? A bug you hit, a feature you need,
+         tech debt, or a prod issue?
+       - The pain being addressed — user-facing problem, or what it unblocks.
+     For non-trivial features, please open an issue/discussion first to align on
+     scope before writing code. -->
+
+
+## What changed
+
+<!-- Describe the change from a user's / caller's perspective, not as a code diff. e.g.:
+       - "Settings now has a 'Custom endpoint' field, off by default"
+       - "Backend /api/chat gains a `stream` flag, defaults to false"
+       - "Default model changed from X to Y — existing users notice on first run" -->
+
+
+## Surface area
+
+<!-- Check every box that applies. Reviewers use this to scope the review. -->
+
+- [ ] **Frontend UI** — page / component / setting / interaction under `frontend/`
+- [ ] **Backend API** — endpoint / SSE event / request-response shape under `backend/app`
+- [ ] **Agents / LangGraph** — agent node, graph wiring, `langgraph.json`, or prompt change
+- [ ] **Sandbox** — `docker/` or sandboxed execution
+- [ ] **Skills** — change under `skills/`
+- [ ] **Dependencies** — new/upgraded entry in `backend/pyproject.toml` or `frontend/package.json` (say what it buys us)
+- [ ] **Default behavior change** — changes existing behavior without the user opting in (default model, default setting, data shape)
+- [ ] **Docs / tests / CI only** — no runtime behavior change
+
+
+## Screenshots / Recording
+
+<!-- If you checked "Frontend UI", attach screenshots showing the entry point —
+     where users discover the change — not just the feature in isolation.
+     Before/after is best for behavior changes. Short GIFs welcome. -->
+
+
+## Bug fix verification
+
+<!-- Skip (delete) this section if this PR is not a bug fix.
+
+     Bugs should be encoded as a failing test that goes red before the fix.
+     Confirm:
+       - Test path that reproduces the bug:
+       - Did it go red on `main` and green on this branch? (yes / no)
+       - If a red test wasn't cheap to write, explain why and what you did instead. -->
+
+
+## Validation
+
+<!-- What you actually ran. Run at least the checks for the area you changed:
+       Backend:   cd backend  && make lint && make test
+       Frontend:  cd frontend && pnpm format && pnpm lint && pnpm typecheck && BETTER_AUTH_SECRET=local-dev-secret pnpm build && make test
+       Frontend E2E (if you touched frontend/): cd frontend && make test-e2e -->
+