[feature] Automate cherry-picking of [fix] commits to the stable branch

[nemesifier]

Currently, commits labeled with the [fix] prefix on the master branch are manually cherry-picked into the current release’s bugfix branch (e.g., 1.1). This process is error-prone and time-consuming.

I propose introducing a GitHub Action to automate this:

• Detect [fix] commits on master/main.
• If marked for backport, e.g., with a label or keyword like [backport], automatically:
  • Cherry-pick the commit into a new branch, eg: backport/1.1/2025-08-07.
  • Open a pull request targeting the stable branch (eg: 1.1).
  • Maintainers then can merge if the CI build passes, if not they'll have to manually resolve.

This will help us to waste less time on backporting fixes and reduce the chance of missing bug fixes in patch releases.

Benefit: Faster, safer, and more consistent backporting of important bug fixes.

We should do this after #496.