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/constants.ts

@@ -10,6 +10,7 @@ export const DEFAULT_MAX_TOOL_CALLS = 4000
 export const DEFAULT_CIRCUIT_BREAKER_CONSECUTIVE_THRESHOLD = 20
 export const DEFAULT_CIRCUIT_BREAKER_ENABLED = true
 export const MIN_RUNTIME_BEFORE_STALE_MS = 30_000
+export const DEFAULT_SESSION_GONE_TIMEOUT_MS = 60_000
 export const MIN_IDLE_TIME_MS = 5000
 export const POLLING_INTERVAL_MS = 3000
 export const TASK_CLEANUP_DELAY_MS = 10 * 60 * 1000