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

db/schema.sql

@@ -309,11 +309,20 @@ CREATE TABLE IF NOT EXISTS activity (
     created_at         TIMESTAMPTZ NOT NULL DEFAULT now()
 );
 ALTER TABLE activity ADD COLUMN IF NOT EXISTS metadata JSONB NOT NULL DEFAULT '{}';
+-- main_seg segments the main-agent session into resettable conversations: a new
+-- main session bumps the segment so its transcript + activity start clean while the
+-- task's graph/assets/goal are untouched. NULL == legacy rows == segment 0 (the
+-- original session). Only worker='mainagent' rows carry it.
+ALTER TABLE activity ADD COLUMN IF NOT EXISTS main_seg INTEGER;
 CREATE INDEX IF NOT EXISTS idx_act_node  ON activity(exploration_id, node_id, id);
 CREATE INDEX IF NOT EXISTS idx_act_since ON activity(exploration_id, id);
 -- Main/Plan history pages filter by worker (both carry NULL node_id, so idx_act_node
 -- can't distinguish them); this covers reverse pagination of those sessions.
 CREATE INDEX IF NOT EXISTS idx_act_worker ON activity(exploration_id, worker, id);
+-- Main-session pages filter by segment on top of worker='mainagent'; this partial
+-- index covers reverse pagination within one segment.
+CREATE INDEX IF NOT EXISTS idx_act_main_seg ON activity(exploration_id, main_seg, id)
+    WHERE worker='mainagent';
 -- Task-list polls aggregate result usage and find the latest event repeatedly.
 -- Cover the token columns for index-only aggregation and the timestamp order for
 -- per-exploration latest-activity lookups.
@@ -322,6 +331,18 @@ CREATE INDEX IF NOT EXISTS idx_act_result_usage ON activity(exploration_id)
     WHERE kind='result';
 CREATE INDEX IF NOT EXISTS idx_act_latest ON activity(exploration_id, created_at DESC);
 
+-- main_sessions records the resettable main-agent conversation segments of a task.
+-- Segment 0 (the original session) is implicit and never stored; this table holds
+-- only the extra segments created by "新建会话" (seq >= 1). The current segment is
+-- MAX(seq) or 0. Each segment gets its own transcript file + activity slice; the
+-- task's exploration graph/assets/goal are shared and never reset.
+CREATE TABLE IF NOT EXISTS main_sessions (
+    exploration_id BIGINT NOT NULL REFERENCES explorations(id) ON DELETE CASCADE,
+    seq            INTEGER NOT NULL,
+    created_at     TIMESTAMPTZ NOT NULL DEFAULT now(),
+    PRIMARY KEY (exploration_id, seq)
+);
+
 -- =====================================================================
 -- C. LLM profiles
 -- =====================================================================