Add org-wide pull request template

Codifies the handbook PR description format (Summary/Validation/Tests/
Notes) as the default template GitHub prefills for every repo in the org
that lacks its own. Hidden hint comments carry the title, scope, and Jira
linking conventions so contributors fill in structure instead of a blank
box.

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,18 @@
+<!--
+PR conventions — see engineering-handbook/pull-requests.md
+- Title: imperative mood, under 70 chars, describe the change not the ticket (e.g. "Add receipt parser Lambda", not "PROJ-123" or "Bug fix").
+- Scope: one logical change per PR. If the title needs an "and", split it.
+- Jira: put the issue key in the branch name or this PR title (e.g. [PROJ-123]) to link the PR into the Jira issue's development panel. Omit if the work has no ticket.
+-->
+
+## Summary
+<!-- What changed and why — 1-3 sentences. Explain the motivation, not just the diff. -->
+
+## Validation
+<!-- How you verified it works — steps taken, commands run, screenshots if UI. -->
+
+## Tests
+<!-- What tests were added, updated, or run. If no automated tests, explain the manual testing. -->
+
+## Notes
+<!-- Anything reviewers should know: migration steps, deploy order, follow-ups, breaking changes. Delete this section if empty. -->