Handle local sibling upstreams when resolving PR defaults

[Copilot]

Branches using Git’s remote = . convention were misclassified as non-GitHub remotes, causing repeated PR-default polling errors in stacked-branch workflows.

• Origin resolution

  • Treat . as a local branch relationship.
  • Fall back to the existing GitHub remote selection logic.
  • Preserve errors for actual non-GitHub remotes.

• Regression coverage

  • Verify PR defaults resolve through origin when a branch tracks a local sibling.

• Fixes Blame hover / PR-defaults polling fails with "is not a GitHub repo" when branch upstream is a local sibling branch (stacked-branch workflows) #8888

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[Copilot]

Pull request overview

This PR fixes PR-default resolution when a branch’s upstream is configured as a local sibling via Git’s remote = . convention, preventing the extension from misclassifying the upstream as a non-GitHub remote and repeatedly erroring during PR-default polling in stacked-branch workflows.

Changes:

• Update FolderRepositoryManager.getOrigin to treat upstream remotes with name '.' as a local-branch relationship (skip upstream-remote GitHub resolution and fall back to normal remote selection).
• Add a unit test to ensure PR defaults still resolve through a GitHub remote when the branch tracks a local sibling.

Show a summary per file

File	Description
src/github/folderRepositoryManager.ts	Skip upstream-based origin resolution when upstreamRef.remote === '.', allowing fallback GitHub remote selection.
src/test/github/folderRepositoryManager.test.ts	Adds regression coverage for local-sibling upstream behavior when resolving PR defaults.

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• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Lite