feat(frontend): xterm.js sur WebSocket avec reconnexion et scrollback (#13)

Lot F3 du chantier server/client mode : le client web branche xterm.js sur
le terminal distant via WebSocket, en face de l'endpoint PTY B5.

- wsLiveClient.ts : transport WebSocket du terminal avec reconnexion.
- streamGateways.ts : gateway terminal (open/attach/data/resize/close).
- frames.ts : frames PTY alignées sur le contrat serveur B5.
- Tests : terminalGateway.test.ts, wsLiveClientReconnect.test.ts.

Validé : frontend 743 tests verts (F3 ciblé 12), cohérence des frames
B5↔F3 confirmée, desktop non régressé.

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

frontend/src/adapters/http/frames.ts

@@ -94,7 +94,7 @@ export interface ServerFrame {
   payload: Record<string, unknown>;
 }
 
-/** Payload of a `terminal.attached` acknowledgement. */
+/** Payload of a `terminal.attached` acknowledgement (B5, `server.rs`). */
 export interface AttachedPayload {
   session: {
     sessionId: string;
@@ -104,8 +104,15 @@ export interface AttachedPayload {
     cols: number;
   };
   nextSeq: number;
+  /**
+   * Bounded scrollback replayed at (re)attach. B5 sends a single entry (`seq:0`)
+   * carrying all retained bytes, or an empty array when there is nothing to
+   * replay.
+   */
   scrollback: { seq: number; bytesBase64: string }[];
   gap: boolean;
+  /** Lifecycle status carried on the ack (B5 sends `"running"`). */
+  status?: string;
   /** Conversation id minted by an `agent.launch` (mirrors `assignedConversationId`). */
   assignedConversationId?: string;
 }
@@ -117,6 +124,16 @@ export interface OutputPayload {
   bytesBase64: string;
 }
 
+/**
+ * Payload of a `terminal.status` frame (B5). `status` is a lifecycle transition
+ * (`"exited"` on close); `exitCode` is present for `"exited"` (may be `null`).
+ */
+export interface StatusPayload {
+  sessionId: string;
+  status: string;
+  exitCode?: number | null;
+}
+
 /** Payload of a `chat.output` frame (structured assistant stream). */
 export interface ChatOutputPayload {
   sessionId: string;