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

assets/oh-my-opencode.schema.json

@@ -4661,6 +4661,10 @@
           "type": "number",
           "minimum": 60000
         },
+        "taskTtlMs": {
+          "type": "number",
+          "minimum": 300000
+        },
         "sessionGoneTimeoutMs": {
           "type": "number",
           "minimum": 10000