feat(frontend): surface agent web sur WebSocket (cellule agent) (#13)

Lot F4 du chantier server/client mode : le client web expose une cellule
agent branchée sur le PTY WebSocket, en face de agent.launch (B6).

- wsLiveClient.ts : launchAgent sur le transport WebSocket.
- streamGateways.ts : gateway agent alignée sur le contrat serveur B6.
- WebAgentCell.tsx : cellule agent web, câblée dans WebWorkspace et index.
- Tests : agentGateway.test.ts, WebApp.test.tsx.

Validé : frontend 749 tests verts, contrat B6↔F4 aligné (aucun écart de
frame), desktop non régressé.

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

frontend/src/adapters/http/wsLiveClient.ts

@@ -340,6 +340,38 @@ export class WsLiveClient {
     );
   }
 
+  /**
+   * Launches a CLI agent (`agent.launch`) over the same PTY channel and tracks
+   * it exactly like a terminal (B6): the ack is a unified `terminal.attached`
+   * carrying `sessionId` + `assignedConversationId`, then output/input/resize and
+   * reconnection replay reuse the terminal mechanics. Structured agents are
+   * refused server-side with `UNSUPPORTED` (this channel is PTY-only); the
+   * rejected `send` surfaces that error to the caller unchanged.
+   */
+  launchAgent(params: {
+    projectId: string;
+    agentId: string;
+    rows: number;
+    cols: number;
+    nodeId?: string | null;
+    conversationId?: string | null;
+    onData: (bytes: Uint8Array) => void;
+    onStatus?: (status: string, exitCode?: number | null) => void;
+  }): Promise<TerminalAttachResult> {
+    return this.attachInternal(
+      "agent.launch",
+      {
+        projectId: params.projectId,
+        agentId: params.agentId,
+        nodeId: params.nodeId ?? null,
+        rows: params.rows,
+        cols: params.cols,
+        conversationId: params.conversationId ?? null,
+      },
+      { onData: params.onData, onStatus: params.onStatus, lastSeq: 0 },
+    );
+  }
+
   /** Re-attaches to an existing terminal (`terminal.attach`) and tracks it. */
   attachTerminal(params: {
     sessionId: string;