docs: Add 'AI-assisted contributions' section in CONTRIBUTING.md and PR field

.github/pull_request_template.md

@@ -25,8 +25,27 @@
 - Related issue #\<issue number\>
 - Closes #\<issue number\>
 
+## AI assistance
+
+<!--
+For transparency only.
+By submitting this PR you accept full responsibility for every line of code,
+regardless of how it was produced.
+See [AI-assisted contributions](https://github.com/narwhals-dev/narwhals/blob/main/CONTRIBUTING.md#ai-assisted-contributions).
+-->
+
+- [ ] No AI tools were used for this PR.
+- [ ] AI tools were used.
+
 ## Checklist
 
 - [ ] Code follows style guide (ruff)
 - [ ] Tests added
 - [ ] Documented the changes
+- [ ] If this is your first PR to narwhals, attach a screenshot of `pytest` passing locally (not CI):
+
+    ```bash
+    PYTEST_ADDOPTS="--numprocesses=logical" \
+    make run-ci DEPS="--extra pandas --extra dask --group core-tests --group sklearn --group plugins" \
+    CMD="pytest tests --cov=src --cov=tests --runslow --constructors=pandas,pandas[nullable],pandas[pyarrow],pyarrow,polars[eager],polars[lazy],dask,duckdb,sqlframe"
+    ```

CONTRIBUTING.md

@@ -397,6 +397,36 @@ heavy third-party libraries (pandas, NumPy, Polars, etc...) please follow these
 We're trying to be really lightweight and minimal-overhead, and
 unnecessary imports can slow things down.
 
+## AI-assisted contributions
+
+We do not categorically reject AI-assisted contributions, but we ask for transparency and accountability.
+
+The aim is to keep maintainer time focused on review quality, not on triaging machine-generated noise.
+
+1. **Disclose AI usage in the PR description**: when opening a pull request, the template
+    has a dedicated field that must be flagged to disclose whether or not the changes are
+    AI assisted and which tools and models were used to achieve that.
+    Disclosure is for transparency; it does not affect whether your PR is accepted.
+
+2. **You are responsible for every line you submit, regardless of how it was produced**:
+    the disclosure does not transfer responsibility to the tool.
+    Before opening a PR, read through your diff and make sure you can explain and defend each change in review.
+
+3. **Engage with review feedback**: we reserve the right to close, without extended discussion,
+    pull requests where the author does not engage with reviewer comments.
+
+    **This applies whether or not AI was used.**
+
+4. **First-time contributors** are asked to:
+
+    - Post a screenshot of the test suite passing locally (not on CI) in the PR description.
+      This is a quick check that your dev environment works and that you have run the code you are proposing.
+    - Keep at most one open pull request at a time, so reviewers can give it proper attention before
+      you open the next one.
+
+5. When interacting in issues, pull requests, discussion, discord, etc., **do not use LLMs to speak for you**,
+    except for translation or grammar edits. Human-to-human communication is foundational to open source communities.
+
 ## Claiming issues
 
 If you start working on an issue, it's usually a good idea to let others know about this