feat(chat): valide le routing cellKind=chat côté frontend et corrige UX

- Ajoute cellKind dans AgentChatHandle et vérifie que launchAgentChat reçoit cellKind:chat
- Rejette avec erreur STRUCTURED_ROUTED_TO_PTY si le backend route vers PTY
- Ajoute cellKind:chat dans MockAgentGateway.launchAgentChat
- Corrige l'overflow du layout CustomAgentChatView (bounded shell avec scroll + composer fixe)
- Dédouble les prompts utilisateur quand le stream les echo
- Ajoute les tests launchAgentChat returns handle only when confirms chat et launchAgentChat rejects PTY-routed launch
- Ajoute les tests does not duplicate user prompt when echoed et keeps chat shell bounded

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

frontend/src/adapters/agent.test.ts

@@ -170,4 +170,77 @@ describe("TauriAgentGateway invoke payloads", () => {
     });
     expect(invoke).not.toHaveBeenCalledWith("close_agent_session", expect.anything());
   });
+
+  it("launchAgentChat returns a handle only when launch_agent confirms cellKind chat", async () => {
+    invoke.mockResolvedValueOnce({
+      sessionId: "chat-session-1",
+      cwd: "/repo",
+      rows: 24,
+      cols: 80,
+      cellKind: "chat",
+      assignedConversationId: "conversation-1",
+    });
+
+    const out = await new TauriAgentGateway().launchAgentChat("proj-1", "agent-2", {
+      cwd: "/repo",
+      rows: 24,
+      cols: 80,
+      conversationId: "conversation-0",
+      nodeId: "node-3",
+    });
+
+    expect(invoke).toHaveBeenCalledWith("launch_agent", {
+      request: {
+        projectId: "proj-1",
+        agentId: "agent-2",
+        rows: 24,
+        cols: 80,
+        cellKind: "chat",
+        conversationId: "conversation-0",
+        nodeId: "node-3",
+      },
+      onOutput: expect.anything(),
+    });
+    expect(out).toEqual({
+      sessionId: "chat-session-1",
+      cellKind: "chat",
+      assignedConversationId: "conversation-1",
+    });
+  });
+
+  it("launchAgentChat rejects a PTY-routed launch before returning a fake structured session", async () => {
+    const log = vi.spyOn(console, "error").mockImplementation(() => {});
+    invoke.mockResolvedValueOnce({
+      sessionId: "pty-session-1",
+      cwd: "/repo",
+      rows: 24,
+      cols: 80,
+      cellKind: "pty",
+    });
+
+    await expect(
+      new TauriAgentGateway().launchAgentChat("proj-1", "agent-2", {
+        cwd: "/repo",
+        rows: 24,
+        cols: 80,
+        nodeId: "node-3",
+      }),
+    ).rejects.toEqual({
+      code: "STRUCTURED_ROUTED_TO_PTY",
+      message:
+        "custom CLI launch for agent agent-2 in project proj-1 returned cellKind=pty; expected chat. sessionId=pty-session-1; nodeId=node-3",
+    });
+    expect(log).toHaveBeenCalledWith(
+      "[ticket149] launchAgentChat:routed-to-non-chat",
+      expect.objectContaining({
+        projectId: "proj-1",
+        agentId: "agent-2",
+        response: expect.objectContaining({
+          sessionId: "pty-session-1",
+          cellKind: "pty",
+        }),
+      }),
+    );
+    log.mockRestore();
+  });
 });