feat(ui): sidebar lists workspaces with their sessions, Claude Desktop style

The sidebar's project tree becomes a flat workspace list (WorkspaceTree):
one row per workspace in the stored order, each expanding to its sessions.
A session that still has a live pane opens /workspace?lane=<id>&run=<runId>,
which only attaches pane 0 to that tmux session; any other session opens
its read-only transcript. Nothing in the sidebar starts or resumes a run.

client/src/components/run/LaneConsolePane.tsx

@@ -90,6 +90,9 @@ export interface LaneConsolePaneProps {
    *  `nonce` is a new request; it goes through the same path as a resume from
    *  the run history, live-run fallback included. */
   resumeRequest?: { sessionId: string; model: string | null; nonce: number } | null;
+  /** A live run to show in this pane (a session picked in the sidebar). It
+   *  attaches to that tmux session only — nothing is started or resumed. */
+  attachRequest?: { runId: string; nonce: string } | null;
 }
 
 /** The tmux session a lane's split pane owns (see server/lib/pty-run.js). */
@@ -114,6 +117,7 @@ export function LaneConsolePane({
   hiddenRunIds,
   onHandleChange,
   resumeRequest,
+  attachRequest,
 }: LaneConsolePaneProps) {
   const { t } = useTranslation("run");
   const { t: tLanes } = useTranslation("lanes");
@@ -216,6 +220,29 @@ export function LaneConsolePane({
     refreshList();
   }, [laneId, slot, refreshList]);
 
+  // Runs after the lane-switch effect above, so a sidebar pick that changes
+  // lane AND run at once lands on the picked run, not the lane's own.
+  useEffect(() => {
+    if (!attachRequest) return;
+    let live = true;
+    autoAttachedForLane.current = laneId;
+    api.run
+      .get(attachRequest.runId)
+      .then((h) => live && setHandle(h))
+      .catch(
+        (err: unknown) =>
+          live &&
+          setError(
+            t("errors.attachFailed", { message: err instanceof Error ? err.message : "unknown" })
+          )
+      );
+    return () => {
+      live = false;
+    };
+    // Only a new request (nonce) attaches; a lane switch never replays one.
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [attachRequest?.nonce]);
+
   // Picking a pipeline template writes NOTHING into the prompt box: a prefilled
   // slash command (or the codex stage-checklist prose) is sent as soon as the
   // run starts, which is an extra prompt nobody asked for. The prompt is the

client/src/components/run/__tests__/LaneConsolePane.test.tsx

@@ -526,4 +526,34 @@ describe("LaneConsolePane", () => {
     await screen.findByTestId("terminal-view");
     expect(api.lanes.action).toHaveBeenCalledTimes(1);
   });
+
+  it("attaches to a run picked in the sidebar without starting or resuming anything", async () => {
+    vi.mocked(api.run.get).mockResolvedValue({
+      id: "agenthub-lane-1-2",
+      laneId: 1,
+      slot: 2,
+      status: "running",
+      cwd: "/workspace/a",
+      model: null,
+      permissionMode: null,
+      effort: null,
+      resumeSessionId: null,
+      sessionId: "s-9",
+      startedAt: null,
+      promptPreview: null,
+      tool: "claude",
+    });
+    render(
+      <MemoryRouter>
+        <LaneConsolePane
+          {...baseProps()}
+          attachRequest={{ runId: "agenthub-lane-1-2", nonce: "k1" }}
+        />
+      </MemoryRouter>
+    );
+    await screen.findByTestId("terminal-view");
+    expect(api.run.get).toHaveBeenCalledWith("agenthub-lane-1-2");
+    expect(api.lanes.action).not.toHaveBeenCalled();
+    expect(api.run.start).not.toHaveBeenCalled();
+  });
 });