Mismatch in background agent changed file count after migrating changes to worktree

[bamurtaugh]

Ref https://github.com/microsoft/vscode-internalbacklog/issues/6388

• Have a workspace open with some uncommitted local change to file A
• Delegate a task to background agent
• Choose to migrate uncommitted local change from file A to the background agent's new worktree
• Background agent does work on worktree, and it introduces changes to a second file B too
• Recent Sessions lists that 2 files were changed (A from the local migration, B from the background agent's work itself), which is accurate
  • 
• ❓ The pill in the background chat output and the changed file count above the chat input box both only count 1 file (file B)

(specifically looking at this part of the image)

I think I'd expect all areas to list the same 2 file changed count. Or perhaps the chat output could explain it only changed 1 file (file B), but the summary of changed files about the prompt input box should still total 2.