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.laneRouter.test.tsx

@@ -67,6 +67,13 @@ const h = vi.hoisted(() => {
 const routerApply = h.routerApply;
 let lanesToReturn: LaneFixture[] = [];
 
+// 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>>();
   return {