Fix worktree checkout for pull requests from forks

[Copilot]

Worktree checkout fetched fork PR branches from the base repository’s remote, where they do not exist. The resulting notification hid Git’s underlying error.

• Remote resolution: Reuse the existing fork-remote helper to find or create the correct remote, preserving SSH/HTTPS selection.
• Branch safety: Use unique PR branch names for forks to avoid selecting unrelated local branches such as main. Preserve same-repository checkout behavior.
• Diagnostics: Surface Git stderr in notifications and logs, falling back to the existing error message when unavailable.

• Fixes Checkout Pull Request in Worktree fails for PRs from forks ("Failed to execute git") #8983

[azure-pipelines]

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