community: CODEOWNERS, issue and PR templates, code of conduct, claiming issues; CI on pull requests from forks

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

.github/pull_request_template.md

@@ -0,0 +1,11 @@
+Closes #
+
+## What and why
+
+## How it was verified
+<!-- Tests added or changed; oracle cases (tests/oracle) for solver and meshing results, with their cited references. -->
+
+## Checklist
+- [ ] The issue was assigned to me before I started (see CONTRIBUTING.md)
+- [ ] `cargo fmt --all`, `cargo clippy -p <crates I touched> --all-targets -- -D warnings` and their tests pass
+- [ ] No code, text or data from GPL-family projects or commercial CAE products