fix(digest): 多智能体评审修正 — 资产索引收敛 + 60cap 保 hot + 文档同步

对照 cold-digest-spec 的对抗式评审(20 项发现)逐条核对实现，两处真实代码问题已修：

- 资产索引膨胀(评审 §6.2 高危)：早前把一个 digest 落进它成员命中的【每个】asset 桶，
  跨资产 digest 会复制进几十个桶、顶层条目不降反升(真实图 asset_ids 约 144 个)。改为
  每个 digest 只归一个【代表资产】桶(成员锚定众数、并列取最小 id；无锚定 → 0 号兜底桶)，
  条目数 ≤ digest 数。
- 60 兜底截断误砍活跃上下文(评审 §6.3 高危)：hot(live 前沿祖先链 / live 意图下新事实)
  是活跃推理上下文，绝不能截。改为 hot 全保、60-cap 只约束【冷但未折】残余；冷区两侧
  均衡保留。graph_overview 每轮计算 render 期 hot 集(兼用于复活检查)。

其余 16 项评审发现经核对，本实现已由设计选择规避/处理：全量扫描维护(静默变冷/传递翻转
盖章)、签名含锚节点、minor+major 统一走同一 group()(§3.1 在两处都生效、major 不会拆散
同父组)、提交时活跃度复核(不折入复活成员)、content_version 列+状态变更 bump、渲染期索引
(无 KindIndex 陈旧)、cold_digests 与索引并列(body 仍在顶层)、singleton 只进 recent。

文档 §6.2 同步上述解决口径(body 归属、一 digest 一桶、singleton 归 recent)。

agent/tools_digest.go

@@ -48,28 +48,34 @@ func (t *ToolSet) coldDigestOverview() (digests []map[string]any, index []map[st
 		})
 	}
 
-	// index (§6.2): asset → digests touching it. A digest with no anchored asset
-	// falls into the 0 bucket ("(未锚定资产)"). One digest may list under >1 asset.
+	// index (§6.2): asset → digests. Each digest lands in EXACTLY ONE bucket — its
+	// representative asset = the asset anchored on the most of its members (mode;
+	// tie-break lowest id). This is what makes the index converge: bucketing a digest
+	// into every asset its members touch would duplicate it across dozens of buckets
+	// and blow up the top-level count instead of shrinking it (asset_ids are fine-
+	// grained — a real task has ~144 of them). A digest whose members anchor no asset
+	// falls into the 0 bucket ("(未锚定资产)").
 	byAsset := map[int64]map[int64]bool{} // asset id → set of digest ids
 	assetSet := map[int64]bool{}
 	for dID, ms := range memByDigest {
-		touched := map[int64]bool{}
+		counts := map[int64]int{}
 		for _, m := range ms {
 			for _, a := range assetsByNode[m] {
-				touched[a] = true
+				counts[a]++
 			}
 		}
-		if len(touched) == 0 {
-			touched[0] = true
+		rep, best := int64(0), 0
+		for a, c := range counts {
+			if c > best || (c == best && (rep == 0 || a < rep)) {
+				rep, best = a, c
+			}
 		}
-		for a := range touched {
-			if byAsset[a] == nil {
-				byAsset[a] = map[int64]bool{}
-			}
-			byAsset[a][dID] = true
-			if a != 0 {
-				assetSet[a] = true
-			}
+		if byAsset[rep] == nil {
+			byAsset[rep] = map[int64]bool{}
+		}
+		byAsset[rep][dID] = true
+		if rep != 0 {
+			assetSet[rep] = true
 		}
 	}
 	labels := map[int64]string{}