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)

tests/tasks-graph-layout.spec.ts

@@ -7,6 +7,7 @@
  * the width-aware solver.
  */
 import { describe, expect, it } from 'vitest'
+import type { SidebarChildLiveView } from '../src/context-types.ts'
 import {
   GRAPH_GAP_Y,
   GRAPH_LIVE_H,
@@ -50,6 +51,12 @@ function layoutRow(layout: ReturnType<typeof layoutTasksGraph>, id: string): num
   return (box.y - GRAPH_PAD) / GRAPH_ROW_STRIDE
 }
 
+/** One running node's live view: a merged activity row, no text tail. */
+const ACTIVITY: SidebarChildLiveView = {
+  running: true,
+  summary: { counts: [{ kind: 'read', count: 1 }], running: 'read', runningDetail: 'x' },
+}
+
 describe('layoutTasksGraph', () => {
   it('places the root on row 0 and children one row below', () => {
     const nodes: TasksNode[] = [agent('root'), agent('a', 'root'), agent('b', 'root')]
@@ -98,11 +105,11 @@ describe('layoutTasksGraph', () => {
     expect(plain.boxes.get('mate')?.h).toBe(GRAPH_NODE_H)
   })
 
-  it('reserves the live tail row only when the tail carries a tool AND text', () => {
+  it('reserves the live tail row only when the tail carries activity AND text', () => {
     const nodes: TasksNode[] = [
       agent('root'),
-      agent('tool-only', 'root', false, { live: { tool: { name: 'read', args: 'x' } } }),
-      agent('both', 'root', false, { live: { tool: { name: 'read', args: 'x' }, text: 'y' } }),
+      agent('tool-only', 'root', false, { live: ACTIVITY }),
+      agent('both', 'root', false, { live: { ...ACTIVITY, text: 'y' } }),
     ]
     const layout = layoutTasksGraph(nodes)
     // One live row either way (the tool row, or the meta-styled thinking
@@ -118,9 +125,7 @@ describe('layoutTasksGraph', () => {
     expect(GRAPH_ROW_STRIDE).toBeGreaterThanOrEqual(tallest)
     const nodes: TasksNode[] = [
       agent('root'),
-      agent('busiest', 'root', false, {
-        live: { tool: { name: 'read', args: 'x' }, text: 'y' }, tasks: [task('t1')],
-      }),
+      agent('busiest', 'root', false, { live: { ...ACTIVITY, text: 'y' }, tasks: [task('t1')] }),
     ]
     const layout = layoutTasksGraph(nodes)
     expect(layout.boxes.get('busiest')?.h).toBe(tallest)