SUP-47: Add GitHub PR surfaces to sidebar and pane topbar

[khoi]

Summary

• add a shared GitHub client/coordinator pipeline that discovers git/gh, resolves remotes, looks up PRs, and stores PR snapshots per terminal surface
• add GitHub settings and diagnostics, then render shared PR presentation in the sidebar row and selected pane topbar
• add focused tests for settings persistence, PR lookup behavior, coordinator cadence/coalescing, and sidebar presentation

Testing

• make -C apps/mac lint
• xcodebuild test -workspace apps/mac/supaterm.xcworkspace -scheme supaterm -destination "platform=macOS" -parallel-testing-enabled NO -only-testing:supatermTests/SettingsFeatureTests -only-testing:supatermTests/SupatermSettingsTests -only-testing:supatermTests/SupatermSettingsSchemaTests -only-testing:supatermTests/TerminalSidebarChromeViewTests -only-testing:supatermTests/GithubClientTests -only-testing:supatermTests/GithubPullRequestRefreshCoordinatorTests CODE_SIGNING_ALLOWED=NO CODE_SIGNING_REQUIRED=NO CODE_SIGN_IDENTITY="" -skipMacroValidation
• push hook validation passed, including web checks and the full mac test suite

[khoi]

Closing all open PRs per request.