chore: add PR template (#239)

Add .github/pull_request_template.md matching the standard template used in all other language repos.

Co-authored-by: wphillipmoore-claude <255925739+wphillipmoore-claude@users.noreply.github.com>

Author: wphillipmoore

.github/pull_request_template.md

@@ -0,0 +1,21 @@
+# Pull Request
+
+## Summary
+
+-
+
+## Issue Linkage
+
+- Fixes # (default; use when no acceptance criteria exist)
+- Ref # (use when acceptance criteria exist)
+- Work is not complete until the issue is closed.
+- If no issue exists, open one before any work begins.
+
+## Testing
+
+- markdownlint
+- `./mvnw verify`
+
+## Notes
+
+-