refactor(client): subagent lineage 遍历收敛为共享 subagent-lineage 模块

rootAncestor / countSubagentDescendants / treeSessionIds 三份「沿
subagent-origin 链上行 + seen 集合防环 fail-soft」实现收敛到新模块
src/client/subagent-lineage.ts：核心走链是单个私有生成器
subagentOriginChain()（seen 防环、只在节点为 subagent 且有 parent 时
前行），三个公开函数迁入并基于它重写；isSideThreadSummary 一并迁入
（countSubagentDescendants 依赖它，迁入可避免 detect↔lineage 循环
依赖），subagent-detect.ts / subagent-jobs.ts re-export 全部迁出
符号，既有 import 调用点零变化。

语义逐字保留的等价性论证：
- rootAncestor：链为空返回 start.id（原循环条件不满足即返回
  current?.id）；链非空返回 byId[末节点.parentId]?.id ?? sessionId
  （原循环退出时 current 恰为该行，断链/行缺失回退 sessionId 不变）。
- countSubagentDescendants：生成器在 seen.add 后 yield、消费方判
  parentId === sessionId 即 break——与原循环体内判断顺序一致；
  descendant 的 running 判定、side-thread 过滤位置不变。
- treeSessionIds：根行缺失（byId[rootId] 无行）时任何上行都不可能
  「访问到」根（原实现只匹配真实访问过的行），故提前返回空集；
  根行自身（非 subagent）经 summary.id === rootId 特判入集；其余
  行经链节点 parentId === rootId 判定，等价于原「上行走到 id ===
  rootId 的行」——两种表述指向同一行（链上节点的 next 行）。

directSubagentCount 不迁移：它是对直接子行的单层过滤，没有上行
walk，无共享逻辑（背景侦查将其归入 walk 家族与实际代码不符）。
collectBranchIds 遍历的是 catalog 结构而非 session 链，保持不动。

src/client/subagent-detect.ts

@@ -9,23 +9,20 @@
  * - {@link countSubagentDescendants}: uninterrupted subagent-origin lineage
  *   totals (mirror of the official `indexSubagentDescendants` over the
  *   plugin's own summary rows).
+ *
+ * The lineage walks themselves ({@link isSideThreadSummary}, {@link
+ * rootAncestor}, {@link countSubagentDescendants}) live in
+ * ./subagent-lineage.ts — the single shared walk implementation — and are
+ * re-exported below for their established import sites.
  */
 import type {
   SidebarSessionList,
-  SidebarSessionSummary,
   SidebarSubagentCatalog,
 } from '../context-types.ts'
-import { SIDE_LABEL_PREFIX } from '../sidechat-core.ts'
+import { countSubagentDescendants, isSideThreadSummary, rootAncestor } from './subagent-lineage.ts'
 
-/**
- * Side Chat threads ride the subagent origin (main-list hiding + the RPC
- * ownership fence) but they are NOT subagent topology: they carry the
- * durable 'Side: ' label and live as sidebar tabs. Excluding them here
- * keeps the auto-open trigger and the Subagent page counts clean.
- */
-export function isSideThreadSummary(summary: SidebarSessionSummary): boolean {
-  return summary.origin === 'subagent' && summary.displayTitle.startsWith(SIDE_LABEL_PREFIX)
-}
+export { countSubagentDescendants, isSideThreadSummary, rootAncestor }
+export type { SubagentDescendantTotals } from './subagent-lineage.ts'
 
 /** Count the direct subagent children of one session (durable `origin` rows). */
 export function directSubagentCount(
@@ -40,28 +37,6 @@ export function directSubagentCount(
   return count
 }
 
-/**
- * The main agent of the current session's tree: walk the durable parent
- * chain upward until the first non-subagent session. The Subagent page shows
- * THIS root's full topology regardless of how deep the current selection is
- * (a session whose row is still hydrating, or a broken chain, degrades to
- * the session itself).
- */
-export function rootAncestor(
-  byId: SidebarSessionList['byId'],
-  sessionId: string | undefined,
-): string | undefined {
-  if (sessionId === undefined) return undefined
-  const seen = new Set<string>()
-  let current: SidebarSessionSummary | undefined = byId[sessionId]
-  while (current !== undefined && current.origin === 'subagent'
-    && current.parentId !== undefined && !seen.has(current.id)) {
-    seen.add(current.id)
-    current = byId[current.parentId]
-  }
-  return current?.id ?? sessionId
-}
-
 /**
  * Collect every catalog branch (an entry with `hasChildren`) reachable from
  * the root — the set of catalogs the always-expanded topology consumes.
@@ -101,37 +76,3 @@ export function detectNewDirectSubagent(
   return directSubagentCount(prev.byId, sessionId) === 0
     && directSubagentCount(next.byId, sessionId) > 0
 }
-
-/** Descendant totals of one session through an uninterrupted subagent-origin chain. */
-export interface SubagentDescendantTotals {
-  count: number
-  runningCount: number
-}
-
-/**
- * Index every subagent descendant under each ancestor it reaches through an
- * uninterrupted subagent-origin chain (same semantics as the official
- * `indexSubagentDescendants`; cycles fail soft).
- */
-export function countSubagentDescendants(
-  byId: SidebarSessionList['byId'],
-  sessionId: string,
-): SubagentDescendantTotals {
-  const totals: SubagentDescendantTotals = { count: 0, runningCount: 0 }
-  for (const descendant of Object.values(byId)) {
-    if (descendant.origin !== 'subagent' || isSideThreadSummary(descendant)) continue
-    const seen = new Set<string>()
-    let current: SidebarSessionSummary | undefined = descendant
-    while (current?.origin === 'subagent' && current.parentId !== undefined
-      && !seen.has(current.id)) {
-      seen.add(current.id)
-      if (current.parentId === sessionId) {
-        totals.count += 1
-        if (descendant.running === true) totals.runningCount += 1
-        break
-      }
-      current = byId[current.parentId]
-    }
-  }
-  return totals
-}