Fix Agent Host branch diffs after base updates (#315324)

* Fix agent host branch diffs after base updates

Prefer remote-tracking base refs for Agent Host worktree creation and branch-changes diff computation so stale local base branches do not introduce unrelated upstream changes.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address worktree cleanup in agent host git test

Add a direct filesystem cleanup fallback after removing the temporary worktree so the regression test remains hermetic if git cleanup fails.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>