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/plugin/event.ts

@@ -195,6 +195,7 @@ export function createEventHandler(args: {
 
   const dispatchToHooks = async (input: EventInput): Promise<void> => {
     await Promise.resolve(hooks.autoUpdateChecker?.event?.(input));
+    await Promise.resolve(hooks.legacyPluginToast?.event?.(input));
     await Promise.resolve(hooks.claudeCodeHooks?.event?.(input));
     await Promise.resolve(hooks.backgroundNotificationHook?.event?.(input));
     await Promise.resolve(hooks.sessionNotification?.(input));