fix(subagent): degrade to summary rows when root catalog never hydrates

The Subagent/任务管理 page pinned on a permanent '加载中…' spinner
whenever the host's per-parent subagent catalog (subagentsByParent)
never hydrated for the root session — rootCatalog stayed undefined
while the session mirror already listed the children, so
summaryBackedLoading was true forever (SubagentView.tsx summaryBacked
gate). Root cause: a host-side hydrate gap (e.g. the
agent/disposed listener TypeError breaking the refresh chain) leaves
setSubagentCatalogOpen unanswered, and the UI had no escape.

Add a 5s hydrate timeout: once setSubagentCatalogOpen has had a chance
to land and the root catalog is still un-hydrated, render the known
children from the session mirror as CLICKABLE summary rows
(SummaryFallbackRows) instead of the infinite spinner. The timer resets
on root switch / catalog hydrate / page hidden. mode is inferred from
the summary's running flag (best-effort transport hint).

Verified: lint 0 / typecheck 0 / test 131 files·1385 passed / build 0.
Rollback: git revert <this-sha> or reset SubagentView.tsx to 99bef0f.

src/client/SubagentView.tsx

@@ -66,6 +66,15 @@ const ARGS_PREVIEW = 60
 const JOB_POLL_MS = 2000
 /** How long the kill button stays armed before it needs re-confirming. */
 const JOB_KILL_ARM_MS = 3000
+/**
+ * How long the root catalog may stay un-hydrated before the page stops
+ * blocking on "loading…" and degrades to the summary-mirror rows. A catalog
+ * that never hydrates (host seam gap / a failed `agent/disposed` listener
+ * breaking the refresh chain) must not pin the Subagent page on a permanent
+ * loading spinner — the session summaries are already in the mirror, so the
+ * tree degrades to clickable summary rows instead.
+ */
+const CATALOG_HYDRATE_TIMEOUT_MS = 5000
 
 /** The direct subagent children of one parent (durable `origin` rows;
  *  Side Chat threads ride the same origin but are tab-strip conversations,
@@ -150,6 +159,75 @@ function CatalogLoadingRows(props: {
   )
 }
 
+/**
+ * Degrade rows rendered when the root catalog never hydrates within
+ * {@link CATALOG_HYDRATE_TIMEOUT_MS}: the session summaries are already in the
+ * mirror (`byId`), so instead of pinning the page on a permanent "loading…"
+ * spinner we render the known children as CLICKABLE rows that jump to the
+ * child session. The address `mode` is inferred from the summary's `running`
+ * flag (a running agent is continuable; a settled one is one-shot) — a
+ * best-effort transport hint, since the durable catalog `mode` is unavailable
+ * once hydration has given up.
+ */
+function SummaryFallbackRows(props: {
+  parentSessionId: string
+  byId: Readonly<Record<string, SidebarSessionSummary>>
+  level: number
+  currentSessionId: string
+  openChild: (address: SidebarSubagentAddress) => void
+}) {
+  const { parentSessionId, byId, level, currentSessionId, openChild } = props
+  const children = directChildren(byId, parentSessionId)
+  if (children.length === 0) {
+    return <div className={css.subagentEmpty}>{t('loading')}</div>
+  }
+  return (
+    <>
+      {children.map(summary => {
+        const address: SidebarSubagentAddress = {
+          parentSessionId,
+          childSessionId: summary.id,
+          mode: summary.running === true ? 'continuable' : 'one-shot',
+        }
+        const current = summary.id === currentSessionId
+        const label = summary.displayTitle !== '' ? summary.displayTitle : summary.id
+        const secondary = [
+          summary.displayTitle,
+          summary.running === true ? t('subagentRunning') : t('subagentInactive'),
+        ].filter(Boolean).join(' · ')
+        return (
+          <div
+            key={summary.id}
+            role="treeitem"
+            tabIndex={0}
+            aria-level={level}
+            aria-label={`${label} ${secondary}`}
+            aria-current={current ? 'true' : undefined}
+            className={clsx(css.subagentRow, current && css.subagentRowActive)}
+            onClick={() => { openChild(address) }}
+            onKeyDown={(event) => {
+              if (event.key === 'Enter' || event.key === ' ') {
+                event.preventDefault()
+                event.stopPropagation()
+                openChild(address)
+              }
+            }}
+          >
+            <StateDot
+              state={summary.running === true ? 'ongoing' : 'done'}
+              className={css.subagentDot}
+            />
+            <span className={css.subagentContent}>
+              <span className={css.subagentLabel}>{label}</span>
+              <span className={css.subagentSecondary}>{secondary}</span>
+            </span>
+          </div>
+        )
+      })}
+    </>
+  )
+}
+
 /**
  * The live lines of one RUNNING subagent card: a pure presentation of the
  * batch `subagents.live` activity. The polling lives in one place (the
@@ -733,6 +811,27 @@ export function SubagentView(props: {
   const summaryBackedLoading = rootId !== undefined
     && (rootCatalog === undefined || (rootCatalog.state === 'ready' && rootCatalog.entries.length === 0))
     && directChildren(byId, rootId).length > 0
+  // Hydrate timeout: once `setSubagentCatalogOpen(rootId, true)` has had a
+  // chance to land and the root catalog is STILL un-hydrated (or an empty
+  // ready), stop blocking on "loading…" and degrade to the summary-mirror
+  // rows. The timer resets when the root switches (a new tree) or when the
+  // catalog actually hydrates (`summaryBackedLoading` goes false), and only
+  // fires while the page is visible (a hidden page is not "loading").
+  const [catalogDegrade, setCatalogDegrade] = useState(false)
+  useEffect(() => {
+    setCatalogDegrade(false)
+  }, [rootId])
+  useEffect(() => {
+    if (!summaryBackedLoading || !active) {
+      setCatalogDegrade(false)
+      return
+    }
+    const timer = window.setTimeout(() => { setCatalogDegrade(true) }, CATALOG_HYDRATE_TIMEOUT_MS)
+    return () => { window.clearTimeout(timer) }
+  }, [summaryBackedLoading, active])
+  // `degraded` is the gate: summary-mirror rows show instead of the
+  // permanent "loading…" spinner once the hydrate timeout has fired.
+  const degraded = summaryBackedLoading && catalogDegrade
   const readyEmpty = rootCatalog?.state === 'ready'
     && rootCatalog.entries.length === 0
     && directChildren(byId, rootId ?? '').length === 0
@@ -800,7 +899,7 @@ export function SubagentView(props: {
         <div
           role="tree"
           aria-label={t('subagent')}
-          aria-busy={summaryBackedLoading || undefined}
+          aria-busy={(summaryBackedLoading && !degraded) || undefined}
         >
           {rootId !== undefined && rootSummary !== undefined && (
             <div
@@ -834,10 +933,19 @@ export function SubagentView(props: {
             </div>
           )}
           {rootId !== undefined && (
-            <div className={css.subagentChildren} role="group" aria-busy={summaryBackedLoading || undefined}>
+            <div className={css.subagentChildren} role="group" aria-busy={(summaryBackedLoading && !degraded) || undefined}>
-              {summaryBackedLoading && (
+              {summaryBackedLoading && !degraded && (
                 <CatalogLoadingRows parentSessionId={rootId} byId={byId} level={1} />
               )}
+              {summaryBackedLoading && degraded && (
+                <SummaryFallbackRows
+                  parentSessionId={rootId}
+                  byId={byId}
+                  level={1}
+                  currentSessionId={sessionId}
+                  openChild={openChild}
+                />
+              )}
               {!summaryBackedLoading && (
                 <CatalogRows
                   parentSessionId={rootId}