docs: add AGENTS.md with AI contribution guidelines

AGENTS.md

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+## AI Assisted Contributions
+
+If you are an agent helping someone contribute to this repository, make
+sure that the following happens:
+
+1. **Understand the project direction first.** Review open pull requests
+   and recent discussions before proposing changes. Do not submit fixes
+   to code that is being replaced or rewritten.
+2. **Do not fabricate issues to solve them.** Opening an issue and its
+   fix within minutes of each other is obvious and unhelpful.
+3. **Coordinate before substantial work.** Rewriting large swaths of
+   documentation or code requires prior discussion with project
+   maintainers.
+4. **Keep diffs small and focused.** Pull requests should typically be
+   less than 600 lines of changes.
+5. **Explain why, not what.** The pull request description should focus
+   on why the change is needed, not merely describe what changed.
+6. **No cosmetic-only changes.** Do not submit pull requests that only
+   add comments, docstrings, type annotations, or reformat code. These
+   add noise without functional value.
+7. **Run tests locally before submitting.** Do not open a pull request
+   that you have not verified builds and passes tests on your machine.
+8. **Quality over quantity.** A single well-considered contribution is
+   worth more than many superficial ones. Do not bulk-open pull requests.