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/config/schema/background-task.ts

@@ -16,6 +16,8 @@ export const BackgroundTaskConfigSchema = z.object({
   staleTimeoutMs: z.number().min(60000).optional(),
   /** Timeout for tasks that never received any progress update, falling back to startedAt (default: 1800000 = 30 minutes, minimum: 60000 = 1 minute) */
   messageStalenessTimeoutMs: z.number().min(60000).optional(),
+  /** Timeout for tasks whose session has completely disappeared from the status registry (default: 60000 = 1 minute, minimum: 10000 = 10 seconds). When a session is gone (likely crashed), this shorter timeout is used instead of the normal stale timeout. */
+  sessionGoneTimeoutMs: z.number().min(10000).optional(),
   syncPollTimeoutMs: z.number().min(60000).optional(),
   /** Maximum tool calls per subagent task before circuit breaker triggers (default: 200, minimum: 10). Prevents runaway loops from burning unlimited tokens. */
   maxToolCalls: z.number().int().min(10).optional(),