fix(directory-explorer): clear selections on Finder pick; dedup VS Code batch

Two findings from the openchamber-bot review at 478f1e9c:

1. Pending selections silently swallowed the Open-in-Finder pick
   (prev #4). handleOpenInFinder flows into the batch-first branch of
   finalizeSelection, so with checkboxes ticked the OS pick was
   ignored and the selections were added instead. Clear selectedPaths
   before finalizeSelection so the Finder-sourced target is honored.

2. Within-batch dedup was missing in the VS Code branch (nit from
   the previous review). A path repeated within one batch hit
   addWorkspaceFolder twice. Mirror the non-VS Code contract with
   a seen Set; add a regression test asserting the host is called
   once per unique path.

packages/ui/src/components/session/DirectoryExplorerDialog.tsx

@@ -555,6 +555,9 @@ export const DirectoryExplorerDialog: React.FC<DirectoryExplorerDialogProps> = (
         return;
       }
 
+      // Clear pending selections so the Finder-sourced target is honored
+      // instead of silently being absorbed by the batch branch.
+      setSelectedPaths([]);
       await finalizeSelection(result.path);
     } catch (error) {
       toast.error(t('directoryExplorerDialog.toast.failedToSelectDirectory'), {