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/streamGateways.ts

@@ -52,22 +52,9 @@ import type {
 } from "@/ports";
 import type { HttpInvoker } from "./httpInvoker";
 import { WsLiveClient } from "./wsLiveClient";
-import { base64ToBytes, type AttachedPayload, type ChatOutputPayload } from "./frames";
+import { type ChatOutputPayload } from "./frames";
 import { unsupportedOnWeb } from "./unsupported";
 
-/** Concatenates a scrollback frame list into a single byte buffer. */
-function attachedToScrollback(payload: AttachedPayload): Uint8Array {
-  const chunks = payload.scrollback.map((c) => base64ToBytes(c.bytesBase64));
-  const total = chunks.reduce((n, c) => n + c.length, 0);
-  const out = new Uint8Array(total);
-  let offset = 0;
-  for (const c of chunks) {
-    out.set(c, offset);
-    offset += c.length;
-  }
-  return out;
-}
-
 /**
  * Builds a {@link TerminalHandle} whose control operations are WS frames. The
  * output stream is delivered through the sink the gateway registered on the
@@ -235,34 +222,34 @@ export class HttpAgentGateway implements AgentGateway {
     options: OpenTerminalOptions,
     onData: (bytes: Uint8Array) => void,
   ): Promise<TerminalHandle> {
-    // Raw-CLI agents stream over the same WS PTY channel (B0). Structured
-    // sessions do not use this channel — that branch is B6.
-    const ack = await this.ws.send("agent.launch", {
+    // B6: a raw-CLI agent streams over the same PTY channel as a terminal. The
+    // WS client tracks the session (reconnection/replay/status parity with F3);
+    // the unified `terminal.attached` ack yields the sessionId + the conversation
+    // id minted by this launch. Structured agents are refused server-side with
+    // `UNSUPPORTED` — the rejection propagates to the caller (a clear cell error,
+    // no crash). A fresh launch has empty scrollback (no repaint on open).
+    const res = await this.ws.launchAgent({
       projectId,
       agentId,
       nodeId: options.nodeId ?? null,
       rows: options.rows,
       cols: options.cols,
       conversationId: options.conversationId ?? null,
+      onData,
     });
-    const payload = ack.payload as unknown as AttachedPayload;
-    const sessionId = payload.session.sessionId;
-    this.ws.setOutputSink(sessionId, onData);
-    const scrollback = attachedToScrollback(payload);
-    if (scrollback.length > 0) onData(scrollback);
-    return makeWsTerminalHandle(sessionId, this.ws, payload.assignedConversationId);
+    return makeWsTerminalHandle(res.sessionId, this.ws, res.assignedConversationId);
   }
 
   async reattach(
     sessionId: string,
     onData: (bytes: Uint8Array) => void,
   ): Promise<ReattachResult> {
-    const ack = await this.ws.send("terminal.attach", { sessionId, lastSeq: null });
-    const payload = ack.payload as unknown as AttachedPayload;
-    this.ws.setOutputSink(sessionId, onData);
+    // Agent sessions re-attach through the same `terminal.attach` mechanics as a
+    // terminal — no relaunch, bounded scrollback replayed for the view to repaint.
+    const res = await this.ws.attachTerminal({ sessionId, onData });
     return {
-      handle: makeWsTerminalHandle(sessionId, this.ws),
-      scrollback: attachedToScrollback(payload),
+      handle: makeWsTerminalHandle(res.sessionId, this.ws),
+      scrollback: res.scrollback,
     };
   }
 }