feat: IM式跨端实时同步——message_persisted 广播 + 前端按id去重追加

- history.Store 新增 AppendFull（返回含 id/createdAt 的完整消息），Append 签名不变
- session_orchestrator 加可空 OnPersisted；persistUserMessages/persistAssistantReply 落库后回调广播
- server.go newOrchestrator 接 OnPersisted → hub.broadcast message_persisted（thinking 剥除）
- 前端 ws.ts 加 message_persisted case（仅 activeSessionId 匹配才处理）；appendOrUpdateMessage 升级为 id 精确upsert+内容/时间归一
- 实测：双 WS，A stream 发消息，B 免刷新收到 user+assistant 各1条（count=2 PASS）
- 红线: 身份仍靠 actor 元数据，不靠提示词；用户/助手消息、本端/异端统一路径

frontend/web/src/lib/ws.ts

@@ -1,7 +1,7 @@
 import { useAppStore } from "../store/useAppStore";
 import { useModeStore } from "../store/useModeStore";
 import { getAuthToken } from "./api";
-import { MODES, type AgentInfo, type AwakenState, type FlowGraph, type Mode, type ToolUsed, type WsUpEvent } from "./types";
+import { MODES, type AgentInfo, type AwakenState, type ChatMessage, type FlowGraph, type Mode, type ToolUsed, type WsUpEvent } from "./types";
 import { now, uid } from "./util";
 
 /**
@@ -156,6 +156,28 @@ function parseGraph(payload: unknown): FlowGraph | null {
   return { id: typeof r.id === "string" ? r.id : "graph", nodes, edges };
 }
 
+/** Build a ChatMessage from a `message_persisted` payload (design/16 §3.3/§4.2):
+ *  the backend broadcast a fully-persisted message carrying the backend-generated
+ *  id. mode is validated against the known modes so a bad value never reaches the
+ *  UI; createdAt falls back to now when the wire omits it. */
+function parsePersistedMessage(v: unknown): ChatMessage | null {
+  const r = toRecord(v);
+  if (!r) return null;
+  const id = typeof r.id === "string" && r.id ? r.id : "";
+  const role = r.role;
+  if (!id || (role !== "user" && role !== "assistant" && role !== "system")) return null;
+  return {
+    id,
+    sessionId: typeof r.sessionId === "string" && r.sessionId ? r.sessionId : undefined,
+    role,
+    content: typeof r.content === "string" ? r.content : "",
+    mode: typeof r.mode === "string" && MODES.includes(r.mode as Mode) ? (r.mode as Mode) : undefined,
+    mood: typeof r.mood === "string" ? r.mood : undefined,
+    image: typeof r.image === "string" ? r.image : undefined,
+    createdAt: typeof r.createdAt === "number" ? r.createdAt : now(),
+  };
+}
+
 function dispatch(ev: { type: string; data: unknown }): void {
   const s = useAppStore.getState();
   const r = toRecord(ev.data);
@@ -191,6 +213,17 @@ function dispatch(ev: { type: string; data: unknown }): void {
       // do NOT end the stream here (stream_end owns that).
       break;
     }
+    case "message_persisted": {
+      // design/16 §4.2 — a user or assistant message was persisted and broadcast
+      // with its backend id. Only surface messages for the active session; the
+      // store dedups by id and adopts the local optimistic bubble (uid → backend
+      // id), so both the originating and remote browsers converge on one message.
+      const msg = parsePersistedMessage(r?.message);
+      if (msg && msg.sessionId === s.activeSessionId) {
+        s.appendOrUpdateMessage(msg);
+      }
+      break;
+    }
     case "thinking.start": {
       pendingThinking = "";
       break;