docs: add AGENTS.md with CLAUDE.md symlink

AGENTS.md

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+# mkdocs-marimo
+
+MkDocs plugin that renders reactive marimo cells in Markdown docs. Published to PyPI as `mkdocs-marimo`.
+
+## Development
+
+Tooling is driven by [pixi](https://pixi.sh) (all tasks defined in `pixi.toml`):
+
+```bash
+pixi run postinstall           # editable install into the env
+pixi run test                  # pytest
+pixi run mypy mkdocs_marimo    # type check
+pixi run pre-commit-run        # lint/format (ruff, taplo, typos) — matches CI lint job
+pixi run docs                  # serve docs locally (pixi run docs-build to build)
+```
+
+- CI runs `test` + `mypy` across the `py39`–`py313` environments (`pixi run -e py312 test`).
+- No pixi? These map to `pytest`, `mypy`, and `pre-commit run -a` under the env deps in `pixi.toml`.

CLAUDE.md

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+AGENTS.md