Add agent contribution guidance

.github/PULL_REQUEST_TEMPLATE.md

@@ -7,3 +7,12 @@ Fixes #
 - [ ] Documentation updated
 - [ ] Changelog entry added
 - [ ] `./check.sh` passed
+
+### AI assistance disclosure
+
+- [ ] No AI assistance was used.
+- [ ] AI assistance was used partially or completely.
+
+If AI assistance was used, describe what it was used for:
+
+(...)

AGENTS.md

@@ -0,0 +1,57 @@
+# Agent Guidance
+
+These instructions apply to the entire repository.
+
+## Project Layout
+
+- Runtime code lives in `src/mss`.
+- Tests live in `src/tests`.
+- Documentation lives in `docs/source`.
+- `check.sh` and `check.ps1` are the maintainer-provided quality check entry points.
+
+## Working Guidelines
+
+- Keep changes focused on the issue or PR scope.
+- Be careful with public API changes. Update docs and tests when behavior changes.
+- Prefer the existing ctypes-based, dependency-free style for runtime code.
+- Do not add runtime dependencies unless the issue explicitly calls for one.
+- If AI assistance was used, disclose it in the pull request template and state which parts were AI-assisted.
+
+## Validation
+
+Install development dependencies:
+
+```shell
+python -m pip install -e ".[dev,tests]"
+```
+
+Run quality checks before submitting a PR:
+
+```shell
+./check.sh
+```
+
+On Windows:
+
+```powershell
+.\check.ps1
+```
+
+Run tests:
+
+```shell
+python -m pytest
+```
+
+On headless GNU/Linux environments, run tests with a virtual display:
+
+```shell
+xvfb-run python -m pytest
+```
+
+When documentation changes, install docs dependencies and build docs with warnings as errors:
+
+```shell
+python -m pip install -e ".[docs]"
+sphinx-build -d docs docs/source docs_out --color -W -bhtml
+```