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.ts

@@ -18,6 +18,7 @@ import type {
   Agent,
   AgentContextDocument,
   EffortSelection,
+  GatewayError,
   ReplyChunk,
   ResumableAgent,
   TerminalSession,
@@ -42,10 +43,28 @@ interface LaunchAgentResponse {
   cwd: string;
   rows: number;
   cols: number;
+  /** Backend-derived routing: `chat` for structured sessions, `pty` for native TUI. */
+  cellKind?: "chat" | "pty";
   /** Conversation id minted by this launch (omitted when nothing was assigned). */
   assignedConversationId?: string;
 }
 
+function structuredLaunchRoutedToPtyError(
+  projectId: string,
+  agentId: string,
+  options: OpenTerminalOptions,
+  response: LaunchAgentResponse,
+): GatewayError {
+  const actual = response.cellKind ?? "missing";
+  return {
+    code: "STRUCTURED_ROUTED_TO_PTY",
+    message:
+      `custom CLI launch for agent ${agentId} in project ${projectId} ` +
+      `returned cellKind=${actual}; expected chat. ` +
+      `sessionId=${response.sessionId}; nodeId=${options.nodeId ?? "none"}`,
+  };
+}
+
 export class TauriAgentGateway implements AgentGateway {
   listAgents(projectId: string): Promise<Agent[]> {
     return invoke<Agent[]>("list_agents", { projectId });
@@ -194,13 +213,32 @@ export class TauriAgentGateway implements AgentGateway {
         agentId,
         rows: options.rows,
         cols: options.cols,
+        cellKind: "chat",
         conversationId: options.conversationId ?? null,
         nodeId: options.nodeId ?? null,
       },
       onOutput: channel,
     });
+    if (res.cellKind !== "chat") {
+      const error = structuredLaunchRoutedToPtyError(projectId, agentId, options, res);
+      console.error("[ticket149] launchAgentChat:routed-to-non-chat", {
+        projectId,
+        agentId,
+        request: {
+          rows: options.rows,
+          cols: options.cols,
+          cellKind: "chat",
+          conversationId: options.conversationId ?? null,
+          nodeId: options.nodeId ?? null,
+        },
+        response: res,
+        error,
+      });
+      throw error;
+    }
     return {
       sessionId: res.sessionId,
+      cellKind: "chat",
       ...(res.assignedConversationId
         ? { assignedConversationId: res.assignedConversationId }
         : {}),