fix(agent): 主 Agent 每个会话均可交互，修实时徽章滞留

上一版把非当前段设为只读、且仅当前段可发消息，与"像会话页一样在任一
会话都能对话"的诉求不符；同时实时徽章改用 recentLive 后，turn 结束仍
会在 5s 窗口内继续显示"实时流式中"。

- server: chat 接口接收目标段 seg，消息写到用户正在看的会话段（缺省=最新段）
- web: 去掉只读/当前/历史区分，任一主 Agent 会话都可直接对话；send 带 seg
- web: 实时徽章改回按真实运行状态(sending/mainChatRunning)判断，且收到
  终结记录(result/报错)即刻熄灭；输入框同样在终结记录后立即解禁

web/src/lib/api.ts

@@ -721,8 +721,8 @@ export const api = {
     if (!r.ok) throw new Error(`report: ${r.status}`);
     return r.text();
   },
-  chat: (message: string, task?: string, attachments?: ChatAttachment[]) =>
-    post<{ reply: string; mode: string }>(`/chat${tq(task)}`, { message, attachments }),
+  chat: (message: string, task?: string, attachments?: ChatAttachment[], seg?: number) =>
+    post<{ reply: string; mode: string }>(`/chat${tq(task)}`, { message, attachments, seg }),
   chatStatus: (taskId: string) => get<{ running: boolean }>(`/tasks/${taskId}/chat/status`),
   // 方式1 文件上传:落到会话/任务工作目录 uploads/，返回可供 agent Read 的相对路径。
   chatUpload: async (scope: "task" | "session" | "staging", id: string, files: File[]) => {

web/src/lib/types.ts

@@ -831,7 +831,6 @@ export interface Session {
   source_task_id?: string;
   inherited?: boolean;
   seg?: number; // main-agent session: which conversation segment (0 = original)
-  isCurrentMain?: boolean; // main-agent session: the writable current segment
 }
 
 // ---- Security ----