refactor(graph_overview): 概览各列表加数量上限，止住上下文膨胀

用"最新一窗 + 计数/pull 兜底"替换 hot/cold 拆分与 balancedCap：
- recent_facts 20、recent_done_intents 12、open_intents 30(按 priority)
- cold_digests 15，按最新成员时间(max member id)降序，溢出的更旧 digest
  只给裸 id(cold_digests_more)，仍可 expand_digest 展开
- 新增 CountOpenIntents，frontier_open 报开放意图真实总数(与截断视图解耦)
- 删除 balancedCap/unfoldedCap/unfolded_truncated 及其测试

掉出窗口的节点不丢：facts/done_intents_total/frontier_open/findings_total
计数在，list_facts/list_findings/node_detail/expand_digest 可按需回捞

db/exploration.go

@@ -808,6 +808,16 @@ WHERE exploration_id=$1 AND kind='intent' AND state IN ('done','blocked','exhaus
 	return n, err
 }
 
+// CountOpenIntents counts this exploration's open intents — graph_overview's
+// frontier_open, so the planner knows open_intents (capped at the top-N by
+// priority) is a truncated view and there may be more claimable work.
+func (s *ExplorationStore) CountOpenIntents() (int, error) {
+	var n int
+	err := s.db.QueryRow(`SELECT COUNT(*) FROM exploration_nodes
+WHERE exploration_id=$1 AND kind='intent' AND state='open'`, s.expID).Scan(&n)
+	return n, err
+}
+
 // GetNode returns one node of this exploration by id (nil, nil if not found).
 func (s *ExplorationStore) GetNode(id int64) (*Node, error) {
 	n, err := scanNode(s.db.QueryRow(`SELECT `+nodeCols+` FROM exploration_nodes WHERE id=$1 AND exploration_id=$2`, id, s.expID))