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/hooks/index.ts

@@ -53,3 +53,4 @@ export { createJsonErrorRecoveryHook, JSON_ERROR_TOOL_EXCLUDE_LIST, JSON_ERROR_P
 export { createReadImageResizerHook } from "./read-image-resizer"
 export { createTodoDescriptionOverrideHook } from "./todo-description-override"
 export { createWebFetchRedirectGuardHook } from "./webfetch-redirect-guard"
+export { createLegacyPluginToastHook } from "./legacy-plugin-toast"