refactor(tasks): 移植主 Agent 同款合并活动折叠（数据层）

把上一轮验证过的数据层搬到工作图基座上，图/树两种模式共用：

- 新增 src/process-activity.ts：宿主 ui-chat 的 process group 算法移植
  （activity() 分类表逐条照抄、liveToolDetail 键优先级与 160 字截断、
  counts 计数排序与并列规则、preparing 语义、按 user/reply 边界切范围）。
  按本轮设计裁剪掉逐阶段列表（阶段改由徽标 + 颜色表达）。新增一条
  偏差：最新范围为空（子代理刚回复完）时回退到上一个「做过事」的范围，
  否则已结算卡片的小条只会显示「已完成分析」。
- chatT：从宿主 chat 命名空间读 message.stepProcess.* 措辞（方法调用 +
  try/catch 兜底，key 未命中即回退），4 个回归用例钉住「不许解绑 bind」。
- subagents.live 载荷换新：SidebarChildLiveView { running?, text?, summary?,
  lastEventTime? }，报告树内每个子节点（含已结算）与拓扑根；删除
  expand/stages 机制；childActivity → childLive（「不在 map 里 = 未运行」
  的读法随旧协议一起作废）。
- tasks-model 消费新形状；LiveLine 与节点详情浮窗改显示合并活动措辞。
- 删除 src/subagent-activity.ts 与 tests/subagent-activity.spec.ts。

验证：typecheck / lint 通过；pnpm test 1252 passed / 9 skipped。

(cherry picked from commit 5e5284f214846b0c366048acbee459f3a7a79456)

src/client/api.ts

@@ -7,10 +7,10 @@
  * request). Failures surface as {@link SidebarApiError} with the wire code.
  */
 import { encodeHtmlUrl } from '../html-route.ts'
-import type { LastActivity } from '../subagent-activity.ts'
 import type { SidechatLiveEvent, SidechatLogEvent, SidechatThreadInfo } from '../sidechat-core.ts'
 import type {
   SidebarCreateTeamTaskRequest,
+  SidebarChildLiveView,
   SidebarJobView,
   SidebarSessionEvent,
   SidebarTeamMemberView,
@@ -119,8 +119,8 @@ export interface JobOutputResult {
   read: boolean
 }
 
-/** The `subagents.live` response: running child id → latest activity. */
-export type SubagentLiveResult = { live: Record<string, LastActivity> }
+/** The `subagents.live` response: one row per tree child (and the root). */
+export type SubagentLiveResult = { live: Record<string, SidebarChildLiveView> }
 
 /** The `workflows.list` response: the tree's folded workflow runs. */
 export type WorkflowsListResult = { runs: WorkflowRunView[] }
@@ -375,7 +375,8 @@ export const api = {
   /**
    * One batch live-preview fetch for the whole Subagent tree. The payload is
    * the already-resolved topology ROOT (not a session scope); the host
-   * enumerates descendants once and folds running children's activity.
+   * enumerates descendants once and folds every child's newest process range
+   * into the main agent's merged-activity summary.
    */
   subagentsLive: (rootSessionId: string, signal?: AbortSignal) =>
     call<SubagentLiveResult>('subagents.live', { rootSessionId }, signal),