chore: add "AI Contributions" section to CONTRIBUTING (#9909)

Co-authored-by: Ari Perkkiö <ari.perkkio@gmail.com>

Author: sheremet-va

CONTRIBUTING.md

@@ -94,6 +94,18 @@ VITEST_MODULE_DIRECTORIES=/node_modules/,/packages/
 
 - Use `pnpm run lint:fix` to format files according to the project guidelines.
 
+## AI Contributions
+
+The team welcomes the use of AI as a personal assistant when contributing to Vitest. However, we strongly believe that a real person must be behind every issue and pull request.
+
+All issues and pull requests must be opened by a real person using the official templates. If AI assisted in creating a pull request, please disclose the tool used (e.g. Claude, Codex, Copilot).
+
+Pull requests or issues entirely generated by AI with no human involvement (e.g. by an automated agent) will be labeled "maybe automated" by the maintainers and closed automatically after 3 days unless a real person responds.
+
+AI-generated comments on issues, pull requests or discussions that add no value or contain incorrect information will be hidden by the maintainers.
+
+These measures help reduce maintenance burden and keep the team's work efficient.
+
 ## Maintenance Guidelines
 
 > The following section is mostly for maintainers who have commit access, but it's helpful to go through if you intend to make non-trivial contributions to the codebase.