fix(files): a workspace whose folder was deleted leaves the Files places

The lane payload carries folder_missing (ENOENT/ENOTDIR only). Files hides
such a workspace and never reopens it as the default place; the Workspace
sidebar strikes its name through with a hint to remove it from its menu.
A folder that cannot be read no longer keeps the previous folder's entries
under its error, and a change in Files re-reads the workspace list.

server/__tests__/lanes-lib.test.js

@@ -337,6 +337,16 @@ describe("lanes lib", () => {
     );
   });
 
+  it("payload says folder_missing only when the folder is gone", () => {
+    const fs = require("node:fs");
+    const dir = fs.mkdtempSync(pathMod.join(os.tmpdir(), "lane-gone-"));
+    const l = lanes.createLane({ cwd: dir });
+    assert.equal(lanes.lanePayload(lanes.getLane(l.id)).folder_missing, false);
+    fs.rmSync(dir, { recursive: true });
+    assert.equal(lanes.lanePayload(lanes.getLane(l.id)).folder_missing, true);
+    lanes.deleteLane(l.id);
+  });
+
   it("payload carries node states and progress", () => {
     const l = lanes.createLane({ cwd: "/tmp/wt/d" });
     lanes.setStage(l.id, { stage: "review" });