feat: add PR template with risk assessment (#837)

* feat: add PR template with risk assessment and review tracks

Add a pull request template that includes summary, change type,
validation checklist, security/database impact sections, blast radius,
and rollback plan. Update CONTRIBUTING.md with review track definitions
(A/B/C) based on change risk level.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

* fix: expand CONTRIBUTING.md with setup, workflow, and guidelines

Add getting started, development workflow, code style summary,
database change guidance, and dependency management sections.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

---------

Co-authored-by: Claude Opus 4.6 <[email protected]>

.github/pull_request_template.md

@@ -0,0 +1,50 @@
+## Summary
+
+<!-- 2-5 bullet points: what changed and why -->
+
+-
+
+## Change Type
+
+<!-- Check one -->
+
+- [ ] Bug fix
+- [ ] New feature
+- [ ] Refactor
+- [ ] Documentation
+- [ ] CI/Infrastructure
+- [ ] Security
+- [ ] Dependencies
+
+## Linked Issue
+
+<!-- Closes #N, or "None" -->
+
+## Validation
+
+<!-- How did you verify this works? -->
+
+- [ ] `cargo fmt`
+- [ ] `cargo clippy --all --benches --tests --examples --all-features`
+- [ ] Relevant tests pass: <!-- list specific tests -->
+- [ ] Manual testing: <!-- describe what you tested -->
+
+## Security Impact
+
+<!-- Does this change affect: permissions, network calls, secrets, file access, tool execution, sandbox policy? If yes, describe. If no, write "None". -->
+
+## Database Impact
+
+<!-- Does this add/modify migrations, change schema, or affect both PostgreSQL and libSQL? If yes, describe. If no, write "None". -->
+
+## Blast Radius
+
+<!-- What subsystems does this touch? What could break? -->
+
+## Rollback Plan
+
+<!-- How to revert if this causes problems? For Track C changes, this is mandatory. -->
+
+---
+
+**Review track**: <!-- A (docs/tests/chore) | B (feature/refactor) | C (security/runtime/DB/CI) -->