enforce a branch naming convention

Add a gh workflow to enforce the following branch name syntax:

```
[feat|bug|doc]/[issue-num]-{short_descr_with_underscores}

eg

feat/1234-add_the_thing
bug/1234-empty_pkgfam
doc/1234-add_pkgtesting_page
```

The action would also check that the issue number refers to an existing and open issue.

Use a GitHub Actions workflow as the entry point and first check how workflows are organized in the repository. The completed workflow should enforce the feat/bug/doc branch format, require a numeric issue reference with an underscored short description, and verify that the referenced issue exists and is open.

Written by the indexing model from the issue text.