feat(ui): cellule agent stoppé — état explicite + reopen from scratch (#197, QA verte)

Surface UI du LayoutGrid pour un agent de cellule stoppé:

- Affichage d'un état explicite quand runtimeState vaut « stopped » ou « failedStart », avec actions refresh / reopen from scratch (freshStart) et état restarting pendant la relance.

- theme.css: styles .stopped-agent-cell (boutons primary/secondary, états disabled).

Consomme le contrat runtime issu de #196 (AgentRuntimeState stopped/failedStart, freshStart) déjà présent dans develop; aucune modification runtime ici.

Tests QA verts: LayoutGrid.test.tsx, LayoutGrid.chat.test.tsx (rejoué aussi: agent_lifecycle launch_fresh_start_ignores_existing_conversation_id_and_mints_new_one).

frontend/src/features/layout/LayoutGrid.chat.test.tsx

@@ -51,6 +51,7 @@ import {
   MockProfileGateway,
   MockSystemGateway,
   MockTerminalGateway,
+  MockWorkStateGateway,
 } from "@/adapters/mock";
 import { DIProvider } from "@/app/di";
 import { leaves } from "./layout";
@@ -222,6 +223,75 @@ describe("LayoutGrid custom agent CLI (#147)", () => {
     expect(screen.queryByText(/déjà actif dans une autre cellule/)).toBeNull();
   });
 
+  it("relaunches a stopped custom CLI agent from scratch (#197)", async () => {
+    const layout = new MockLayoutGateway();
+    const agent = new MockAgentGateway();
+    const profileGateway = new MockProfileGateway();
+    const terminal = new MockTerminalGateway();
+    const system = new MockSystemGateway();
+    const workState = new MockWorkStateGateway();
+    await profileGateway.configureProfiles([structuredProfile]);
+    const created = await agent.createAgent("p1", {
+      name: "Worker",
+      profileId: structuredProfile.id,
+    });
+    const tree = await layout.loadLayout("p1");
+    const leafId = leaves(tree)[0].id;
+    await layout.mutateLayout("p1", {
+      type: "setCellAgent",
+      target: leafId,
+      agent: created.id,
+    });
+    await layout.mutateLayout("p1", {
+      type: "setSession",
+      target: leafId,
+      session: "dead-structured-session",
+    });
+    await layout.mutateLayout("p1", {
+      type: "setCellConversation",
+      target: leafId,
+      conversationId: "old-structured-conversation",
+    });
+    window.localStorage.setItem(`idea.agent-cell-mode.p1.${leafId}`, "custom");
+    workState._setProjectWorkState("p1", {
+      agents: [
+        {
+          agentId: created.id,
+          name: "Worker",
+          profileId: structuredProfile.id,
+          runtimeState: "stopped",
+          busy: { state: "idle" },
+        },
+      ],
+    });
+    const launchChat = vi.spyOn(agent, "launchAgentChat");
+
+    renderGrid({
+      layout,
+      agent,
+      profile: profileGateway,
+      terminal,
+      system,
+      workState,
+    } as unknown as Gateways);
+
+    fireEvent.click(await screen.findByRole("button", { name: "Relancer" }));
+
+    await waitFor(() => expect(launchChat).toHaveBeenCalledTimes(1));
+    expect(launchChat.mock.calls[0]?.[2]).toMatchObject({
+      cwd: "/home/me/proj",
+      nodeId: leafId,
+      freshStart: true,
+    });
+    expect(launchChat.mock.calls[0]?.[2].conversationId).toBeUndefined();
+    await waitFor(async () => {
+      const updated = await layout.loadLayout("p1");
+      const leaf = leaves(updated)[0];
+      expect(leaf.session).toMatch(/^mock-agent-chat-/);
+      expect(leaf.conversationId).toMatch(/^mock-conversation-/);
+    });
+  });
+
   it("closes the structured chat session by id before switching back to native TUI (#170)", async () => {
     const layout = new MockLayoutGateway();
     const agent = new MockAgentGateway();