Introduction of an AI-policy for stan-dev repos

ai_policy/AI_policy.md

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+# AI Contribution Policy
+
+We welcome AI-assisted contributions. This policy exists to ensure that all contributions remain the work of an accountable human contributor and that our community has the transparency it needs to collaborate effectively.
+
+## Human Accountability
+
+Every pull request (PR) must be submitted by a named individual who assumes full responsibility for the contribution. While we recognize that automation and AI can be part of a modern workflow, we do not accept fully automated or agentic submissions.
+
+## Licensing and Copyright
+
+Every contributor must agree to the [Developer Certificate of Origin (DCO) 1.1](https://developercertificate.org) within the GitHub PR template. This certifies that each contributor has the right to submit the code under the project's open-source license. This applies regardless of how the code was produced. It is the contributor's responsibility to ensure that any AI-assisted output is compatible with the project's license and that they hold or have cleared the rights to submit it.
+
+## Mandatory AI Use Disclosure
+To maintain transparency and trust within our community, contributors are required to disclose whether AI tools were used in preparing their contribution. We included a corresponding mandatory section in the GitHub PR template. If AI tools were used, the contributor additionally affirms that they have reviewed every change, fully understand the logic and security implications, and are prepared to explain or defend the technical decisions therein during the review process.
+
+### Acknowledgement
+This policy was inspired by the [CloudNativePG AI Policy](https://github.com/cloudnative-pg/governance/blob/main/AI_POLICY.md) and developed with the practical guidance and support of [NumFOCUS](https://numfocus.org) in creating the initial draft. In the spirit of transparency and the "Disclosure" section of this policy, an initial draft of this document was generated using Google Gemini and was subsequently reviewed, edited, and approved by the Stan maintainers.
+
+---
+
+This policy is effective as of [date]. It will be reviewed periodically as AI tools and community norms evolve.
+
+Questions or suggestions? Join the discussion in our [community channel](https://discourse.mc-stan.org).

ai_policy/PULL_REQUEST_TEMPLATE.md

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+# Pull Request
+
+> 💡
+> **Does your PR contain only a trivial change?**
+> 
+> If your PR consists of minor updates, such as fixing a typo, updating a link,
+> or making small documentation tweaks, you can use our **[Lightweight PR Template](?template=trivial-pr.md)** to save time.
+>
+> (Note: Switch to Preview mode to click the link. It will redirect you to open a new PR using the shorter template.)
+
+## Compliance & AI Disclosure
+> This section is mandatory. Do not delete this section. PRs that leave it blank will not be reviewed.
+
+- [ ] I have read the [Contributing Guidelines](../blob/main/.github/CONTRIBUTING.md).
+- [ ] I accept the [Developer Certificate of Origin](https://developercertificate.org).
+- [ ] I have read and comply with the [AI Contribution Policy](../blob/main/AI_policy.md).
+
+**AI Tool Usage Disclosure:**
+*Did you use AI tools (e.g. GitHub Copilot, ChatGPT, Claude, Cursor, etc.) for this PR?*
+
+- [ ] **No AI used.** This PR consists entirely of my own original work.
+- [ ] **AI-Assisted (Minor).** AI was used for boilerplate, syntax, or autocomplete.
+- [ ] **AI-Generated (Major).** AI generated significant logic or entire sections.
+
+> **Note:** If AI was used, I affirm that I have reviewed every line, fully understand the logic and security implications, and am prepared to explain or defend the technical decisions therein during the review process.
+
+## Pre-Review Checklist
+> Please ensure these steps are completed before requesting a review.
+
+- [ ] **Coordination:** For non-trivial changes, an issue was opened and approved by a maintainer. (Link: #____)
+- [ ] **Alignment:** The PR delivers the specific result agreed upon in the linked issue.
+- [ ] **Sync:** Branch is up-to-date with `main` / `develop`.
+- [ ] **Quality:** All existing tests pass locally, and I have added new tests for my changes.
+- [ ] **Docs:** Documentation (README, inline comments, or changelogs) has been updated.
+- [ ] **Notebooks:** If applicable, all `.qmd` / `.Rmd` files render end-to-end in a clean environment without machine-specific paths.
+---
+
+## Summary
+
+<!-- Why is this change needed? What problem does it solve? (1-3 sentences.) -->
+
+Closes # <!-- issue number -->
+
+### Type of Change
+
+- [ ] Bug fix
+- [ ] New feature
+- [ ] Breaking change (requires behaviour change/migration)
+- [ ] Refactoring (no functional change)
+- [ ] Documentation / Tooling / CI
+
+## What Changed
+
+<!--
+  Describe the technical changes in enough detail for a reviewer to understand
+  the approach without reading every line of code.
+  Tip: if you cannot explain a section without re-reading AI output, that section
+  needs more review before this PR is ready.
+-->
+
+-
+-
+-
+
+## Expected Result
+
+<!--
+  Describe concretely what "done" looks like for this PR.
+  This should match what was agreed upon in the linked issue.
+  Include screenshots, logs, or demo output where helpful.
+-->
+
+**Additional Context**
+
+<!-- Links to design docs, related PRs, Slack discussions, etc. -->