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/features/web/WebAgentCell.tsx

@@ -0,0 +1,76 @@
+/**
+ * Web agent cell — ticket #13, lot F4. Thin wiring (no new terminal logic) that
+ * reuses the existing, transport-neutral {@link TerminalView} to render a CLI
+ * agent over the WebSocket in web mode.
+ *
+ * The agent's CLI runs server-side (B6); the browser is display-only. This
+ * component just supplies `TerminalView` with the DI **agent gateway** as the
+ * opener/reattacher:
+ *  - `open`  → `agent.launchAgent(projectId, agentId, …)` (frame `agent.launch`,
+ *    unified `terminal.attached` ack; the minted conversation id is carried on the
+ *    returned handle),
+ *  - `reattach` → `agent.reattach(sessionId, …)` (frame `terminal.attach`, no
+ *    relaunch, bounded scrollback repainted) — so a browser reload/reconnect
+ *    resumes the surviving server-side PTY.
+ *
+ * The session id is persisted in component state so a re-mount re-attaches rather
+ * than relaunching, matching the desktop cell's lifecycle. A structured agent is
+ * refused server-side (`UNSUPPORTED`) and surfaced by `TerminalView`'s own error
+ * banner — no crash. It touches only gateways via DI (no `@tauri-apps/api`).
+ */
+
+import { useCallback, useState } from "react";
+
+import type {
+  OpenTerminalOptions,
+  ReattachResult,
+  TerminalHandle,
+} from "@/ports";
+import { useGateways } from "@/app/di";
+import { TerminalView } from "@/features/terminals";
+
+interface WebAgentCellProps {
+  /** Owning project id (resolved server-side by `agent.launch`). */
+  projectId: string;
+  /** Agent to launch/attach. */
+  agentId: string;
+  /** Working directory for the cell (typically the project root). */
+  cwd: string;
+  /** Layout leaf id, when the caller tracks one (drives the singleton guard). */
+  nodeId?: string;
+}
+
+export function WebAgentCell({ projectId, agentId, cwd, nodeId }: WebAgentCellProps) {
+  const { agent } = useGateways();
+  const [sessionId, setSessionId] = useState<string | null>(null);
+
+  const open = useCallback(
+    (options: OpenTerminalOptions, onData: (bytes: Uint8Array) => void): Promise<TerminalHandle> =>
+      agent.launchAgent(
+        projectId,
+        agentId,
+        nodeId ? { ...options, nodeId } : options,
+        onData,
+      ),
+    [agent, projectId, agentId, nodeId],
+  );
+
+  const reattach = useCallback(
+    (sid: string, onData: (bytes: Uint8Array) => void): Promise<ReattachResult> =>
+      agent.reattach(sid, onData),
+    [agent],
+  );
+
+  return (
+    <div data-testid="web-agent-cell" className="h-64 w-full">
+      <TerminalView
+        cwd={cwd}
+        agentMode
+        open={open}
+        reattach={reattach}
+        sessionId={sessionId}
+        onSessionId={setSessionId}
+      />
+    </div>
+  );
+}