Auto-close linked issues when external PRs are merged via bot

[IrvingMg]

Description

When an external PR includes closing keywords (e.g. Fixes #N), the referenced issue is not automatically closed after merge.

This happens because external PRs are first merged into an intermediate branch (main-pr<N>) instead of main. GitHub only auto-closes issues when changes are merged into the default branch.
See: https://docs.github.com/en/issues/tracking-your-work-with-issues/using-issues/linking-a-pull-request-to-an-issue#about-linked-issues-and-pull-requests

The "Merge External PR" created by the pr-merge.yml workflow merges the intermediate branch into main, but it does not preserve the closing keywords from the original PR body.

Example

PR #545 includes Fixes #207, but after it is merged, issue #207 remains open.

Proposed solution

Update .github/workflows/pr-merge.yml to extract closing keywords (Fixes, Closes, Resolves) from the original PR body and include them in the merge PR body.

Since the merge PR targets main, GitHub should process these keywords and automatically close the referenced issues.

[cmainas]

Hello @IrvingMg ,

yes this is totally valid and annoying. The proposed solution sounds also good.

One general note, not related with your suggestion but related with automatically closing the issues. We need to be more careful with the use of the keywords in the PR descriptions (maintainers should take care of that), because some PRS might reference an issue, but not fully resolve it. In these cases, we should keep the issue open.