feat(delegate-task): ⚙️ make sync subagent timeout configurable via syncPollTimeoutMs

Allow users to set `background_task.syncPollTimeoutMs` in config to override
the default 10-minute sync subagent timeout. Affects sync task, sync continuation,
and unstable agent task paths. Minimum value: 60000ms (1 minute).

Co-authored-by: Wine Fox <fox@ling.plus>

assets/oh-my-opencode.schema.json

@@ -3685,6 +3685,10 @@
         "messageStalenessTimeoutMs": {
           "type": "number",
           "minimum": 60000
+        },
+        "syncPollTimeoutMs": {
+          "type": "number",
+          "minimum": 60000
         }
       },
       "additionalProperties": false