feat(chat): livre la CLI custom de chat agent (#147) et corrige Cancel

Implémente la vue chat structurée par cellule agent (toggle TUI/CLI custom,
préférence persistée `preferred_view`, reattach live, composer + pièces
jointes) avec le socle backend AgentSession/ChatBridge (UserPrompt,
cancel_current_turn, routage interrupt_agent, commande cancel_agent_chat).

Corrige le bug bloquant relevé par QA : le bouton Cancel de
CustomAgentChatView interrompait tout le tour via closeAgentChat au lieu
de n'annuler que le tour courant via cancelAgentChat, ce qui tuait la
session contrairement au contrat produit validé.

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

frontend/src/adapters/agent.ts

@@ -18,15 +18,18 @@ import type {
   Agent,
   AgentContextDocument,
   EffortSelection,
+  ReplyChunk,
   ResumableAgent,
   TerminalSession,
 } from "@/domain";
 import type {
+  AgentChatHandle,
   AgentGateway,
   ConversationDetails,
   CreateAgentInput,
   LiveAgent,
   OpenTerminalOptions,
+  ReattachAgentChatResult,
   ReattachResult,
   StoppedLiveAgent,
   TerminalHandle,
@@ -175,6 +178,69 @@ export class TauriAgentGateway implements AgentGateway {
     };
   }
 
+  async launchAgentChat(
+    projectId: string,
+    agentId: string,
+    options: OpenTerminalOptions,
+  ): Promise<AgentChatHandle> {
+    // `launch_agent` is the routing authority. For structured/headless profiles
+    // the backend ignores the PTY output channel and returns a structured session
+    // id; for non-structured profiles it may still route to PTY, so callers gate
+    // this method on `profile.structuredAdapter`.
+    const channel = new Channel<number[]>();
+    const res = await invoke<LaunchAgentResponse>("launch_agent", {
+      request: {
+        projectId,
+        agentId,
+        rows: options.rows,
+        cols: options.cols,
+        conversationId: options.conversationId ?? null,
+        nodeId: options.nodeId ?? null,
+      },
+      onOutput: channel,
+    });
+    return {
+      sessionId: res.sessionId,
+      ...(res.assignedConversationId
+        ? { assignedConversationId: res.assignedConversationId }
+        : {}),
+    };
+  }
+
+  async reattachAgentChat(
+    sessionId: string,
+    onChunk: (chunk: ReplyChunk) => void,
+  ): Promise<ReattachAgentChatResult> {
+    const channel = new Channel<ReplyChunk>();
+    channel.onmessage = onChunk;
+    return invoke<ReattachAgentChatResult>("reattach_agent_chat", {
+      sessionId,
+      onReply: channel,
+    });
+  }
+
+  async sendAgentChat(
+    sessionId: string,
+    prompt: string,
+    onChunk: (chunk: ReplyChunk) => void,
+  ): Promise<void> {
+    const channel = new Channel<ReplyChunk>();
+    channel.onmessage = onChunk;
+    await invoke("agent_send", {
+      sessionId,
+      prompt,
+      onReply: channel,
+    });
+  }
+
+  async closeAgentChat(sessionId: string): Promise<void> {
+    await invoke("close_agent_session", { sessionId });
+  }
+
+  async cancelAgentChat(sessionId: string): Promise<void> {
+    await invoke("cancel_agent_chat", { sessionId });
+  }
+
   async reattach(
     sessionId: string,
     onData: (bytes: Uint8Array) => void,