docs: add PR template

Adds a structured PR template requiring change description, motivation,
type classification, test plan, and an optional AI disclosure field.
Part of the Phase 1 community management improvements (MUL-320).

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,32 @@
+## What
+
+<!-- What does this PR do? Keep it to 1-3 sentences. -->
+
+## Why
+
+<!-- Why is this change needed? Link the related issue. -->
+
+Closes #<!-- issue number -->
+
+## Type of Change
+
+- [ ] Bug fix
+- [ ] New feature
+- [ ] Refactor / code improvement
+- [ ] Documentation
+- [ ] CI / infrastructure
+- [ ] Other (describe below)
+
+## How to Test
+
+<!-- How can a reviewer verify this works? Steps, commands, or screenshots. -->
+
+## Checklist
+
+- [ ] `make check` passes (typecheck, unit tests, Go tests, E2E)
+- [ ] Changes follow existing code patterns and conventions
+- [ ] No unrelated changes included
+
+## AI Disclosure (optional)
+
+<!-- If AI tools were used, briefly note which ones (e.g., Claude Code, Copilot). -->