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:
People can keep pushing commits to their local branch whenever they want, even if the corresponding PR(s) are closed. We don't care about that.

On the conditions of auto-closing, we can iterate and see what works. Note that we can have different labels for different sets of conditions. Some ideas off the top of my head:

- If there are no new commits
- If the CI stays red
- If the linter CI stays red
- If the OP doesn't respond in terms of a comment, (and maybe with a specific text in it)
- If the OP doesn't push a commit with a specific commit message

after deciding on the conditions, if they're not satisfied within 2 weeks, the PR will be auto-closed.

This has the benefit of completely ignoring PRs with those labels, instead of checking whether they can now be closed or not, periodically.

Labels: Needs Decision, RFC