Add an AI Policy #3632

Opening as an issue to track progress, following the Discord thread discussion.

Proposed measures

• Mandatory AI disclosure on PRs. Requires adding a PR-template field asking whether any AI tool or model was used, and which one. The point is transparency. Addressed by docs: Add 'AI-assisted contributions' section in CONTRIBUTING.md and PR field #3701
• Responsibility clause in CONTRIBUTING.md. Add wording along the lines of:

  You are responsible for every line you submit, regardless of how it was produced. Disclosure does not transfer responsibility to the tool.

• Author responsiveness as the bar: PRs where the author does not engage with review feedback are closed without extended discussion. This is reactive, not preventive: we cannot screen for it upfront, but we can be explicit that PRs where the author does not respond to review feedback will be closed without extended discussion, whether or not AI was used. Addressed by docs: Add 'AI-assisted contributions' section in CONTRIBUTING.md and PR field #3701
• Add an AGENTS.md (under discussion) - WIP in docs: Add AGENTS.md  #3821
• Ship a user-facing SKILL.md files. Examples from polars itself: https://github.com/polars-inc/skills.
• AI-assisted PR review as a supplement to, and never a replacement for, human review. Open questions are:
  • Cost: Do any vendors offer a free allowance for open source projects worth investigating?
    • coderabbit Pro plan is free for Open Source projects
    • qodo also free for OSS
  • If individual maintainers use AI personally for review, we could add a review.md describing what to look for and how to structure the review.
  • Edit: my personal stand on this is that, although useful, these kind of reviews should not run by default, but only when triggered. The main risk I see is biasing the reviewer and/or give a false sense of trust

Additional references:

• Thread originating PR: feat: add Selector.__xor__ and Expr.__xor__ for symmetric difference #3621
• Related human-contributor improvements: chore: Adopt uv as package manager #3608, feat: Introduce testing.constructors module and a pytest plugin #3552
• A community-maintained Survey of OSS AI policies: melissawm/open-source-ai-contribution-policies
• Polars contributing guide: https://docs.pola.rs/development/contributing/
• AGENTS.md standard: https://agents.md/
• Library skills pattern: https://library-skills.io/