Summary

• Fix desktop topic/session trash cleanup when a session transcript or sidecar disappears before or during the move. Previously this could surface the raw Windows error The system cannot find the path specified and leave the sidebar topic feeling stuck.
• Make trash cleanup idempotent: if the source path is already gone, treat that artifact as already removed and continue moving any remaining artifacts. Real blocking errors, such as inaccessible destinations or busy live sessions, still propagate.
• When the source vanishes mid-copy, drop the partially written destination before treating the artifact as already moved, so the trash never keeps a truncated transcript that a later restore would resurrect. Real copy failures still propagate and leave the source untouched.
• Add regression coverage for copy fallback missing-source races, for removing a stale topic whose session path no longer exists, and for partial-destination cleanup when the source vanishes mid-copy.

Verification

• cd desktop && go test . -run "Test(MovePathIfExistsCopyFallback|CopyFallbackTreatsMissingSourceAsAlreadyMoved|CopyFallbackRemovesPartialTargetWhenSourceVanishesMidCopy|CopyFallbackKeepsErrorWhenSourceStillExists|TrashTopicRemovesStaleMissingSession|TrashTopicMovesRelatedSessionsToTrash|TrashTopicValidTrashRemovesEmptyLiveStub)"
• cd desktop && go test . -run "Test(DeleteSessionFile|MovePathIfExists|CopyAndRemove|CopyFallback|TrashTopic|Restore.*Session|PurgeTrashedSession)"
• cd desktop && go test .
• git diff --check

Cache impact

Cache-impact: none, desktop session cleanup only; this does not change provider-visible prompts, tool schemas, request serialization, or model context construction.
Cache-guard: cd desktop && go test . plus focused session trash regression tests listed above.
System-prompt-review: N/A