fix(background-agent): detect stale tasks that never received progress updates

Tasks with no progress.lastUpdate were silently skipped in
checkAndInterruptStaleTasks, causing them to hang forever when the model
hangs before its first tool call. Now falls back to checking startedAt
against a configurable messageStalenessTimeoutMs (default: 10 minutes).

Closes #1769

src/features/background-agent/constants.ts

@@ -4,6 +4,7 @@ import type { BackgroundTask, LaunchInput } from "./types"
 export const TASK_TTL_MS = 30 * 60 * 1000
 export const MIN_STABILITY_TIME_MS = 10 * 1000
 export const DEFAULT_STALE_TIMEOUT_MS = 180_000
+export const DEFAULT_MESSAGE_STALENESS_TIMEOUT_MS = 600_000
 export const MIN_RUNTIME_BEFORE_STALE_MS = 30_000
 export const MIN_IDLE_TIME_MS = 5000
 export const POLLING_INTERVAL_MS = 3000