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.
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