feat(agent): 任务内主 Agent 支持多会话，可切换且上下文可重置

主 Agent 会话原为一个任务恒定单例，对话历史随 transcript 只增不减、
上下文越滚越大。现引入会话分段(seq)：新建会话切一个干净的 transcript
段，任务的图谱/资产/目标共享保留，旧段以只读历史随时切回查看。

- db: 新增 main_sessions 表与 activity.main_seg 列(ALTER ADD COLUMN
  IF NOT EXISTS 兼容旧库)；ActivitySessionFilter 按段过滤(段 0 兼容
  旧 NULL 行)；新增 CurrentMainSeg/ListMainSessions/NewMainSession；
  per-session token 统计按 main:<seg> 分段
- agent: MainAgent.Chat 增加 mainSeg，transcript SessionID 分段
  (段 0 保留旧名 exp<id>-main 兼容既有对话)
- server: parseActivitySession 支持 main:<seq>；chat handler 每轮
  stamp 当前段；新增 GET main-sessions / POST main-session/new
- web: 主 Agent 分组渲染多段会话，支持新建(轻确认)、切换、旧段只读

web/src/lib/api.ts

@@ -678,6 +678,16 @@ export const api = {
     }));
   },
 
+  // Main-agent conversation segments of a task. Each segment is a resettable session
+  // (clean transcript/context) over the same task; `current` is the writable one.
+  mainSessions: (task: string) =>
+    get<{ sessions: { seq: number; created_at: string }[]; current: number }>(
+      `/exploration/main-sessions${tq(task)}`,
+    ).then((r) => ({ sessions: arr(r.sessions), current: r.current ?? 0 })),
+  // Start a fresh main-agent session segment (does not touch the task's graph/assets/goal).
+  newMainSession: (task: string) =>
+    post<{ seq: number; created_at: string; current: number }>(`/exploration/main-session/new${tq(task)}`),
+
   // ---- traffic / audit / report / chat ----
   audit: (task?: string) => get<Audit>(`/audit${tq(task)}`),
   traffic: (page = 0, size = 100, host = "", method = "", q = "") =>

web/src/lib/types.ts

@@ -627,6 +627,7 @@ export interface Activity {
   };
   source_task_id?: string;
   inherited?: boolean;
+  main_seg?: number; // main-agent conversation segment (present only on worker="mainagent" rows)
   // token usage (present only on kind='result')
   input_tokens?: number;
   output_tokens?: number;
@@ -829,6 +830,8 @@ export interface Session {
   intent_id?: string;
   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 ----