ci: limit pull requests to maintainers and an allowlist of invited authors

.github/pull_request_template.md

@@ -2,6 +2,9 @@
 Target branch: 2.x features and fixes go to `studio`. 1.x takes bug fixes,
 provider/API compatibility, release/updater, security and platform stability
 only, on `main-v2`.
+
+Pull requests are limited to maintainers and invited contributors; one from
+anyone else is closed automatically. Open an issue instead.
 -->
 
 ## Summary