Auto-Assign PRs when Issue "Lead" set

### Feature Request

When a contributor open a PR before we set a Lead on the issue, the PR is unassigned... And then once we set the Lead of the issue, the PR does not get automatically updates (i.e. must be updated manually).

This feature proposes a github action to listen for updating/adding of the `Lead` label and updating the corresponding PR w/ assignee

Related to #9212

### Problem / Opportunity

### Proposal

### Breakdown

### Related files

Refer to [this map of common Endpoints](https://github.com/internetarchive/openlibrary/wiki/Endpoints):
*

### Requirements Checklist
Checklist of requirements that need to be satisfied in order for this issue to be closed:
* [ ]

### Stakeholders

*

### Instructions for Contributors

* **Before** [creating a new branch](https://github.com/internetarchive/openlibrary/wiki/Git-Cheat-Sheet#making-changes-and-creating-a-pull-request) or pushing up changes to a PR, please first [run these commands](https://github.com/internetarchive/openlibrary/wiki/Git-Cheat-Sheet#working-on-your-branch) to ensure your repository is up to date, as the pre-commit bot may add commits to your PRs upstream.

Start with related issue #9212 and the linked map of common Endpoints; no implementation files, tests, or entry points are identified. Clarify the GitHub event, Lead-label-to-assignee behavior, and acceptance checklist before selecting an implementation path. Done should mean the corresponding PR is automatically assigned when the issue Lead is added or updated.

Written by the indexing model from the issue text.