Implement a strict "Issue-First" Policy

[KeerthiKumarR]

Hi everyone,

I've noticed that we are getting a lot of unsolicited Pull Requests opened by newcomers, and a significant portion of it ends up being low-quality "AI slop." This is flooding the repository and wasting maintainers' time on reviews.

To stop this PR flooding, I am proposing that we implement a strict "Issue-First Policy".

Under this policy:

1. A contributor cannot raise a PR until they have raised an issue and that issue has been explicitly assigned to them by a maintainer.
2. If a contributor raises a PR without an approved and assigned issue, the PR will be automatically closed.
3. We add this policy clearly to our CONTRIBUTING.md and PULL_REQUEST_TEMPLATE.md so the expectations are set immediately.

This will act as a strong filter against spam PRs and ensure that only maintainer-approved work gets light.

[walterbender]

We have discussed this in the past and hesitated. But the situation is completely out of hand again. I am in favor of restricting new contributors to 1 PR and auto-rejecting that PR if it is not tied to an existing issue.

[walterbender]

By existing issue, I am leaning towards defining it as an issue someone else has raised.

[walterbender]

See #8596 for a specific proposal.

[walterbender]

See #8535