Allow cron jobs to specify a `model_tier` field that controls which LLM
tier handles the job, without needing separate agent configs. Precedence:
cron job model_tier > agent config > global primary_tier > 'default'.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Implements CronScheduler as a ChannelAdapter that fires InboundMessages
on cron schedules and routes agent responses to configured output
channels (e.g. Telegram). Includes 9 tests.