Add a Settings option for GitHub organization scope

[CompleteDotTech]

Context

PR #3700 fixes the incomplete GitHub Organization filter by retrieving the signed-in account's organization memberships. That pull request is intentionally limited to the GitHub surface.

Follow-up

Add a Settings option that lets a user define the GitHub organization scope used by Cave (for example, all memberships or a selected subset). Decide the final UX and persistence model as part of the implementation.

UI ownership

@BunsDev owns the Settings UI and its integration.

Acceptance criteria

• Settings exposes a clear control for the GitHub organization scope.
• The control is populated from the authenticated GitHub membership list.
• The selected scope persists and has a documented reset/default behavior.
• The GitHub surface consistently applies the configured scope.
• PR fix: populate GitHub org filter from memberships #3700 remains limited to the GitHub selector/data correction and does not absorb this Settings feature.

Related work

• GitHub-only fix: fix: populate GitHub org filter from memberships #3700
• Durable work item: cave-lpl

[CompleteDotTech]

@BunsDev FYI: Discussion on conditional filters of Org vs Repo, currently filters out repos that do not have issues/prs that are assigned to you.