Add AI Contribution Policy to CONTRIBUTING.md (#36651)

Based on my recent experience of both using AI tools and reviewing
AI-generated pull requests. Partially based on
https://typescript-eslint.io/contributing/ai-policy/.

---------

Signed-off-by: silverwind <me@silverwind.io>
Co-authored-by: TheFox0x7 <thefox0x7@gmail.com>

CONTRIBUTING.md

@@ -4,6 +4,7 @@
 
 - [Contribution Guidelines](#contribution-guidelines)
   - [Introduction](#introduction)
+  - [AI Contribution Policy](#ai-contribution-policy)
   - [Issues](#issues)
     - [How to report issues](#how-to-report-issues)
     - [Types of issues](#types-of-issues)
@@ -67,6 +68,19 @@ Sensitive security-related issues should be reported to [security@gitea.io](mail
 
 For configuring IDEs for Gitea development, see the [contributed IDE configurations](contrib/ide/).
 
+## AI Contribution Policy
+
+Contributions made with the assistance of AI tools are welcome, but contributors must use them responsibly.
+
+1. Include related issues or pull requests in the prompt so that the AI has ideal context.
+2. Review AI-generated code closely before submitting a pull request.
+3. Manually test the changes and add appropriate automated tests where feasible.
+4. Only use AI to assist in contributions that you understand well enough to respond to feedback without relying on AI.
+5. Indicate AI-generated content in issue and pull requests descriptions and comments. Specify which model was used.
+6. Do not use AI to reply to questions about your issue or pull request. The questions are for you, not an AI model.
+
+Maintainers reserve the right to close pull requests and issues that appear to be low-quality AI-generated content. We welcome new contributors, but cannot sustain the effort of supporting contributors who primarily defer to AI rather than engaging substantively with the review process.
+
 ## Issues
 
 ### How to report issues