fix(#2732): detect crashed subagent sessions with shorter timeout

When a subagent session disappears from the status registry (process
crashed), the main agent was waiting the full stale timeout before
acting. Fix:

- Add sessionGoneTimeoutMs config option (default 60s, vs 30min normal)
- task-poller: use shorter timeout when session is gone from status
- manager: verify session existence when gone, fail crashed tasks
  immediately with descriptive error
- Add legacy-plugin-toast hook for #2823 migration warnings
- Update schema with new config option

src/features/background-agent/task-poller.ts

@@ -7,6 +7,7 @@ import type { OpencodeClient } from "./opencode-client"
 
 import {
   DEFAULT_MESSAGE_STALENESS_TIMEOUT_MS,
+  DEFAULT_SESSION_GONE_TIMEOUT_MS,
   DEFAULT_STALE_TIMEOUT_MS,
   MIN_RUNTIME_BEFORE_STALE_MS,
   TERMINAL_TASK_TTL_MS,
@@ -109,6 +110,7 @@ export async function checkAndInterruptStaleTasks(args: {
     onTaskInterrupted = (task) => removeTaskToastTracking(task.id),
   } = args
   const staleTimeoutMs = config?.staleTimeoutMs ?? DEFAULT_STALE_TIMEOUT_MS
+  const sessionGoneTimeoutMs = config?.sessionGoneTimeoutMs ?? DEFAULT_SESSION_GONE_TIMEOUT_MS
   const now = Date.now()
 
   const messageStalenessMs = config?.messageStalenessTimeoutMs ?? DEFAULT_MESSAGE_STALENESS_TIMEOUT_MS
@@ -122,15 +124,18 @@ export async function checkAndInterruptStaleTasks(args: {
 
     const sessionStatus = sessionStatuses?.[sessionID]?.type
     const sessionIsRunning = sessionStatus !== undefined && isActiveSessionStatus(sessionStatus)
+    const sessionGone = sessionStatuses !== undefined && sessionStatus === undefined
     const runtime = now - startedAt.getTime()
 
     if (!task.progress?.lastUpdate) {
       if (sessionIsRunning) continue
-      if (runtime <= messageStalenessMs) continue
+      const effectiveTimeout = sessionGone ? sessionGoneTimeoutMs : messageStalenessMs
+      if (runtime <= effectiveTimeout) continue
 
       const staleMinutes = Math.round(runtime / 60000)
+      const reason = sessionGone ? "session gone from status registry" : "no activity"
       task.status = "cancelled"
-      task.error = `Stale timeout (no activity for ${staleMinutes}min since start). This is a FINAL cancellation - do NOT create a replacement task. If the timeout is too short, increase 'background_task.staleTimeoutMs' in .opencode/oh-my-opencode.json.`
+      task.error = `Stale timeout (${reason} for ${staleMinutes}min since start). This is a FINAL cancellation - do NOT create a replacement task. If the timeout is too short, increase 'background_task.${sessionGone ? "sessionGoneTimeoutMs" : "staleTimeoutMs"}' in .opencode/oh-my-opencode.json.`
       task.completedAt = new Date()
 
       if (task.concurrencyKey) {
@@ -156,12 +161,14 @@ export async function checkAndInterruptStaleTasks(args: {
     if (runtime < MIN_RUNTIME_BEFORE_STALE_MS) continue
 
     const timeSinceLastUpdate = now - task.progress.lastUpdate.getTime()
-    if (timeSinceLastUpdate <= staleTimeoutMs) continue
+    const effectiveStaleTimeout = sessionGone ? sessionGoneTimeoutMs : staleTimeoutMs
+    if (timeSinceLastUpdate <= effectiveStaleTimeout) continue
     if (task.status !== "running") continue
 
      const staleMinutes = Math.round(timeSinceLastUpdate / 60000)
+     const reason = sessionGone ? "session gone from status registry" : "no activity"
      task.status = "cancelled"
-     task.error = `Stale timeout (no activity for ${staleMinutes}min). This is a FINAL cancellation - do NOT create a replacement task. If the timeout is too short, increase 'background_task.staleTimeoutMs' in .opencode/oh-my-opencode.json.`
+     task.error = `Stale timeout (${reason} for ${staleMinutes}min). This is a FINAL cancellation - do NOT create a replacement task. If the timeout is too short, increase 'background_task.${sessionGone ? "sessionGoneTimeoutMs" : "staleTimeoutMs"}' in .opencode/oh-my-opencode.json.`
      task.completedAt = new Date()
 
     if (task.concurrencyKey) {