feat(agent): robustesse routage ask — fix registre session + concurrence (R0+A0) — §14.3/§15

Stabilise le routage de `ask` avant le bind transport MCP (v5). Invariant
« 1 agent = 1 employé » durci ; un agent traite un tour à la fois.

- R0a garde LaunchAgent : lève AgentAlreadyRunning pour un lancement neuf
  ciblant un agent déjà vivant sur un autre node (PTY + structuré) ; rebind
  seulement même-node ou réattache explicite (conversation_id). Idem spawn_agent.
- R0b list_live_agents agrège PTY + structuré (LiveSessions) + dédup.
- R0c réconciliation des layouts.json à doublons à l'ouverture (host déterministe,
  idempotent) — corrige « une cellule reset au retour d'onglet ».
- R0d UI : option agent désactivée si vivant ailleurs + « aller à la cellule »,
  mapping AGENT_ALREADY_RUNNING.
- A0 sérialisation FIFO des tours par agent_id dans ask_agent (verrou tokio par
  agent ; agents différents en parallèle ; timeout tour 300s, cap attente 600s).

Cadrage : .ideai/briefs/orchestration-v5-transport-bind-cadrage.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/features/layout/singletonAgent.test.tsx

@@ -200,3 +200,94 @@ describe("singleton agent — dropdown guard (T6)", () => {
     expect((option as HTMLOptionElement).disabled).toBe(false);
   });
 });
+
+describe("R0d — dropdown disables agents live elsewhere + go-to-cell", () => {
+  function renderGrid(layout: MockLayoutGateway, agent: MockAgentGateway) {
+    const system = {
+      onDomainEvent: vi.fn().mockResolvedValue(() => {}),
+    };
+    const gateways = { layout, agent, system } as unknown as Gateways;
+    return render(
+      <DIProvider gateways={gateways}>
+        <LayoutGrid projectId="p1" cwd="/home/me/proj" />
+      </DIProvider>,
+    );
+  }
+
+  /** Splits the project's single leaf into two visible leaves; returns both ids. */
+  async function splitInTwo(
+    layout: MockLayoutGateway,
+  ): Promise<{ a: string; b: string }> {
+    const initial = await layout.loadLayout("p1");
+    const a = leaves(initial)[0].id;
+    await layout.mutateLayout("p1", {
+      type: "split",
+      target: a,
+      direction: "row",
+      newLeaf: "b",
+      container: "c",
+    });
+    const after = leaves(await layout.loadLayout("p1"));
+    return { a, b: after.find((l) => l.id !== a)!.id };
+  }
+
+  it("an agent live in another VISIBLE cell ⇒ option disabled + 'aller à la cellule'", async () => {
+    const layout = new MockLayoutGateway();
+    const agent = new MockAgentGateway();
+    const ag = await agent.createAgent("p1", { name: "Busy", profileId: "p" });
+    const { a, b } = await splitInTwo(layout);
+
+    // The agent is live in cell B (a currently-visible leaf).
+    vi.spyOn(agent, "listLiveAgents").mockResolvedValue([
+      { agentId: ag.id, nodeId: b, sessionId: "s-1" },
+    ]);
+
+    renderGrid(layout, agent);
+
+    // In cell A's dropdown the option is DISABLED (cannot run in two cells).
+    const selectA = (await screen.findByLabelText(
+      `agent selector ${a}`,
+    )) as HTMLSelectElement;
+    await waitFor(() => {
+      const opt = Array.from(selectA.options).find((o) => o.value === ag.id)!;
+      expect(opt.disabled).toBe(true);
+      expect(opt.textContent).toMatch(/visible ailleurs/);
+    });
+
+    // Selecting it surfaces a clear notice + an "aller à la cellule" action.
+    fireEvent.change(selectA, { target: { value: ag.id } });
+    expect((await screen.findByRole("status")).textContent).toMatch(
+      /déjà actif dans une autre cellule/,
+    );
+    expect(
+      screen.getByRole("button", { name: "aller à la cellule" }),
+    ).toBeTruthy();
+  });
+
+  it("an agent NOT live anywhere stays selectable (no regression)", async () => {
+    const layout = new MockLayoutGateway();
+    const agent = new MockAgentGateway();
+    const ag = await agent.createAgent("p1", { name: "Free", profileId: "p" });
+
+    // No live agents at all.
+    vi.spyOn(agent, "listLiveAgents").mockResolvedValue([]);
+
+    renderGrid(layout, agent);
+
+    const option = await screen.findByRole("option", { name: "Free" });
+    expect((option as HTMLOptionElement).disabled).toBe(false);
+
+    // And selecting it pins the agent on the cell (no notice, no block).
+    const leafId = leaves(await layout.loadLayout("p1"))[0].id;
+    const select = screen.getByRole("combobox") as HTMLSelectElement;
+    fireEvent.change(select, { target: { value: ag.id } });
+    await waitFor(async () => {
+      const leaf = leaves(await layout.loadLayout("p1")).find(
+        (l) => l.id === leafId,
+      )!;
+      expect(leaf.agent).toBe(ag.id);
+    });
+    expect(screen.queryByRole("status")).toBeNull();
+  });
+
+});