fix(plugins): New workspace commands have the same effects as a pick, even from a mount effect

The screen attached the pick side effects (remembering the isolation, syncing the picker's PR
attachment) as listeners in its own useEffect, which runs after a child item's mount effect. A
mount-time setIsolation("worktree") switched the screen but was not saved, and setBaseRef skipped
the attachment sync. The effects are now injected when the setup model is built, so a command and
a pick run the same code from the first render. Closing the picker stays in the picker's handler.

packages/app/src/plugins/new-workspace-items/model.test.ts

@@ -36,14 +36,16 @@ describe("the base ref a plugin reads", () => {
     const origin = rows.find((row) => row.kind === "branch" && row.refName.includes("remotes"));
     if (!origin) throw new Error("expected an origin row");
 
-    const setup = createNewWorkspaceSetup({
-      locked: false,
-      project: { id: "p", name: "repo" },
-      canCreateWorktree: true,
-      supportsMetadata: true,
-    });
     const chosen: PickerItem[] = [];
-    setup.onBaseChosen((item) => chosen.push(item));
+    const setup = createNewWorkspaceSetup(
+      {
+        locked: false,
+        project: { id: "p", name: "repo" },
+        canCreateWorktree: true,
+        supportsMetadata: true,
+      },
+      { persistIsolation: () => {}, syncBaseAttachment: (item) => chosen.push(item) },
+    );
     const read = toPluginNewWorkspace({
       host: HOST,
       view: viewOf(origin),