fix(#2825): secondary agents no longer pruned after 30 min of total runtime

TTL (pruneStaleTasksAndNotifications) now resets on last activity:
- Uses task.progress.lastUpdate as TTL anchor for running tasks
  (was always using startedAt, causing 30-min hard deadline)
- Added taskTtlMs config option for user-adjustable TTL
- Error message shows actual TTL duration, not hardcoded '30 minutes'
- 3 new tests for the new behavior

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(),
+  /** Absolute TTL for non-terminal tasks in milliseconds (default: 1800000 = 30 minutes, minimum: 300000 = 5 minutes). Tasks exceeding this age from their last activity (or startedAt if no progress) are pruned. */
+  taskTtlMs: z.number().min(300000).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(),