docs: enhance contribution guidelines with issue requirements

CONTRIBUTING.md

@@ -1,5 +1,20 @@
 # Contributing
 
+## Choosing an Issue
+
+To maintain project quality and focus, Playwright **requires a corresponding issue** for every contribution, with the exception of minor documentation fixes.
+
+If you would like to address a bug or feature that isn't currently listed, **please file a new issue first**. This allows the community and maintainers to provide early feedback and facilitates a discussion before you invest time in developing a pull request.
+
+When submitting an issue, please state clearly if you intend to work on it. Once triaged and approved, the maintainers will determine the best path forward—whether the task should be handled by the **core team**, an **automated agent**, or a **community contributor**. If the issue is assigned to you, you may then proceed with your changes and submit a PR.
+
+### Submission Policy
+To ensure the maintainability of the project, please note the following:
+
+* **Unsolicited PRs:** Pull requests submitted without a linked issue or prior approval will be closed.
+* **Low-Quality AI Contributions:** PRs that do not meet our quality standards or lack human oversight (including low-quality agentic submissions) will be closed without explanation.
+* **Approval Required:** Only proceed with a PR once the issue has been officially assigned to you or approved for community contribution.
+
 ## How to Contribute
 
 ### Installing Developer Tools