feat(ui): a Claude-Desktop-style home for every workspace

/workspace?lane=<id> opens on the workspace's home (WorkspaceHome): a prompt
box whose Start opens a new session in the first free pane slot, Recents (a
live session attaches its pane, any other opens its transcript, nothing
resumes), and Instructions / Memory / Context / Folder. Instructions live in
lanes.instructions, extra folders and uploaded files in the new lane_sources
table (copies under <data dir>/workspace-context/<lane id>/); spawnRun reads
them from the lane's rows and hands Claude --add-dir + an appended system
prompt file, Codex --add-dir + developer_instructions. Memory is read-only.

Every dashboard pane carries AGENTHUB_RUN_ID; hooks forward it and the run
records which session the pane holds (a Claude pane follows /clear and
/resume but not a nested claude -p), so the sidebar and notifications open
the right pane (&run=). Settings loses its Projects group; Worktrees now
decides which worktrees Add folder suggests.

client/src/pages/__tests__/Workspace.test.tsx

@@ -50,6 +50,13 @@ let lanesToReturn: LaneFixture[] = [
 let countsToReturn = { total: 2, running: 1, needs_you: 0, dead: 0 };
 let nextRunId = 1;
 
+// The workspace's home (its own suite) fetches sources and memory; these
+// tests are about the session view it hides, so it is a stub here.
+vi.mock("../../components/lanes/WorkspaceHome", () => ({
+  WorkspaceHome: () => <div data-testid="workspace-home" />,
+  paneRunId: (id: number, slot: number) =>
+    slot ? `agenthub-lane-${id}-${slot}` : `agenthub-lane-${id}`,
+}));
 vi.mock("../../lib/api", async (importOriginal) => {
   const actual = await importOriginal<Record<string, unknown>>();
   const recordCall = (method: string, path: string) => {
@@ -694,3 +701,25 @@ describe("split terminal view", () => {
     expect(screen.getAllByTestId("console-body")).toHaveLength(2);
   });
 });
+
+describe("Workspace — home and session view", () => {
+  it("opens a workspace on its home, hiding (not unmounting) the session view", async () => {
+    await renderWorkspace();
+    expect(screen.getByTestId("workspace-home")).toBeInTheDocument();
+    expect(screen.getByTestId("lane-detail").parentElement).toHaveClass("hidden");
+  });
+
+  it("opens the session view for ?run=, and the back button returns home", async () => {
+    render(
+      <MemoryRouter initialEntries={["/workspace?lane=1&run=agenthub-lane-1"]}>
+        <Workspace />
+      </MemoryRouter>
+    );
+    await settle();
+    await settle();
+    expect(screen.queryByTestId("workspace-home")).toBeNull();
+    expect(screen.getByTestId("lane-detail").parentElement).not.toHaveClass("hidden");
+    await userEvent.setup().click(screen.getByTestId("workspace-back-home"));
+    expect(await screen.findByTestId("workspace-home")).toBeInTheDocument();
+  });
+});