RFC: Proposal for autoclose option for non-compliant PRs

Following up on the discussion in #31679 about low-effort contributions (like ai-generated with low human supervision), @lucyleeow and me worked out an idea: We would like to propose introducing an autoclose option for PRs that gets triggered when setting an "autoclose" label that can be removed if the PR is improved.

The idea is to provide a structured way to handle PRs that don't yet meet our contribution standards and facilitate the reviewing process. This will be transparent and fair for new contributors.

**Suggested process**
1. A contributor opens a PR that looks low-effort, fly-by, not supervised ai-generated, or otherwise not yet ready for review.
2. A reviewer sets an "autoclose" label. This would automatically add a polite, standardised comment explaining:
    - The PR will be autoclosed after _n_ weeks (e.g. _4_).
    - A list of common quality issues (similar to what we used in #31822) that outlines what the contributor needs to address in order to avoid the PR being autoclosed (can contain links to the docs and the FAQs where we explain those things in detail).
    - An encouragement to comment on the PR and ping the person who set the "autoclose" once they believe they have met the quality standards and addressed the issues, or when they struggle or need clarification in a specific point.
    - A note that reviewer time is limited and we cannot provide detailed guidance.
3. Possible outcomes:
    - If the author doesn't respond or pushes commits that don't address the issues → PR is autoclosed after the timeout.
    - If the author states problems → reviewers can offer guidance at their discretion, based on capacity and fit.
    - If the author makes the required improvements:
        - ideally either the reviewer who set the label or the person on triage removes the label, which stops the autoclose action, and the PR can be reviewed and move forward as usual
        - in some cases (hopefully not too often) we might miss the deadline and the author would need to open a new PR (that's a downside on applying an the autoclose policy)

**Benefits**
- For maintainers: avoids the implicit obligation to fully review low-quality PRs and allows to focus on genuine contributions.
- For contributors: sets clear expectations from the start, and thoughtful contributors benefit because reviewers have more time for their PRs. 
- For the project: helps keep the PR queue manageable and signals that quality and preparation are expected.

The key point is that this wouldn't reduce our support for new contributors. It only shifts the expectation: authors are responsible for bringing their PRs to a reviewable state, and reviewers can decide how much help to give along the way.

**Technical feasibility**
This should be achievable with a GitHub Action, similar to those used in many open source projects to autoclose stale issues or PRs, but triggered by a label instead of inactivity.

COMMENT:
I am in favour of trying the simplest thing we can think of (for example, no AI-based action) and iterate on it to try to make it useful for us.

Rough plan would be something like this:
- get a setup on a test repo to be able to play with it and test it
- open a PR in scikit-learn and get it merged
- use it for say one month and get feed-back 

For security considerations, I would try to use official Github actions like `actions/stale` as much as possible rather than third-party GitHub actions. If we think in terms of worst outcome, the token would likely need `issue: write` and `pull-request: write`. It would need to be double-checked to be 100% sure, but if you steal such a token you would be able to cause havoc in our GitHub issues/PR, by closing issues, editing commments, probably deleting comments as well.

Labels: Needs Decision, RFC