Add PR template and wire copilot agent to use it

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,19 @@
+## Summary
+
+<!-- Describe what changes were made and why -->
+
+## Validation
+
+<!-- List the commands run to verify the changes -->
+
+```bash
+make compile          # sync triggers + compile to lock files
+make lint             # run all linters
+```
+
+## Pre-Completion Checklist
+
+- [ ] Re-read the issue or request and confirmed this PR directly addresses it
+- [ ] Reviewed all changed files for correctness
+- [ ] Ran `make compile` and `make lint` with no errors
+- [ ] Verified no unrelated files were modified

.github/agents/copilot-coding-agent.md

@@ -58,6 +58,7 @@ Before finishing:
 - Re-read the issue or request one more time and confirm the final response directly addresses it.
 - Double-check changed files and command output for correctness before reporting completion.
 - Prefer a complete, verified solution over a partial fix; if blocked, clearly explain the blocker and what was already verified.
+- When opening a pull request, read `.github/PULL_REQUEST_TEMPLATE.md` and use it as the PR description template, filling in each section.
 
 For this repository: