chore: stabilize main branch with dev workflow + PR guidelines

- Set dev as default branch, protect main (owner-only merge)
- Add CI trigger for PRs targeting dev
- Add PR template with checklist and branch targeting guide
- Add CONTRIBUTING.md with branch strategy and review criteria
- Update README clone command to use -b main for stable

.github/workflows/ci.yaml

@@ -4,7 +4,7 @@ on:
   push:
     branches: [main]
   pull_request:
-    branches: [main]
+    branches: [main, dev]
 
 permissions:
   contents: read