Feature Request: remote repo selector

## Is your feature request related to a problem? Please describe.

Apparently this extension assumes the GitHub repository to use is the `git remote` named `origin`. I use a fork-based development workflow, and I haven't been using `origin` as a remote name at all, as [it's arguably unclear](https://stackoverflow.com/a/38986590). (Does it refer to my fork, or the upstream/canonical repo?) Additionally, even if the meaning of `origin` is clear, this limitation doesn't allow users to choose between multiple remotes.

## Describe the solution you'd like

Add a pulldown selector somewhere in the GITHUB ACTIONS sidepanels, to allow choosing. Save the setting for each workspace.

## Additional context

(None.)

Start by tracing how the extension discovers the current GitHub repository and how the GITHUB ACTIONS sidepanels are assembled. Inspect the workspace settings flow and remote handling first; done means a sidepanel selector can choose among remotes and the choice is saved separately for each workspace.

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