fix(cron): steer model away from explicit deliver targets that lose topic context (#8187)

Rewrite the cronjob tool's 'deliver' parameter description to strongly
guide models toward omitting the parameter (which auto-detects origin
including thread/topic). The previous description listed all platform
names equally, inviting models to construct explicit targets like
'telegram:<chat_id>' which silently drops the thread_id.

New description:
- Leads with 'Omit this parameter' as the recommended path
- Explicitly warns that platform:chat_id without :thread_id loses topics
- Removes the long flat list of platform names that invited construction

Also adds diagnostic logging at two key points:
- _origin_from_env(): logs when thread_id is captured during job creation
- _deliver_result(): warns when origin has thread_id but delivery target
  lost it; logs at debug when delivering to a specific thread

Helps diagnose user-reported issue where cron responses from Telegram
topics are delivered to the main chat instead of the originating topic.

cron/scheduler.py

@@ -219,6 +219,21 @@ def _deliver_result(job: dict, content: str, adapters=None, loop=None) -> Option
     chat_id = target["chat_id"]
     thread_id = target.get("thread_id")
 
+    # Diagnostic: log thread_id for topic-aware delivery debugging
+    origin = job.get("origin") or {}
+    origin_thread = origin.get("thread_id")
+    if origin_thread and not thread_id:
+        logger.warning(
+            "Job '%s': origin has thread_id=%s but delivery target lost it "
+            "(deliver=%s, target=%s)",
+            job["id"], origin_thread, job.get("deliver", "local"), target,
+        )
+    elif thread_id:
+        logger.debug(
+            "Job '%s': delivering to %s:%s thread_id=%s",
+            job["id"], platform_name, chat_id, thread_id,
+        )
+
     from tools.send_message_tool import _send_to_platform
     from gateway.config import load_gateway_config, Platform