feat: add per-cron-job model tier selection

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>

src/automation/cron.ts

@@ -90,7 +90,7 @@ export class CronScheduler implements ChannelAdapter {
       senderName: `cron:${jobName}`,
       text: job.message,
       timestamp: Date.now(),
-      metadata: { cronJob: jobName, scheduled: true },
+      metadata: { cronJob: jobName, scheduled: true, modelTier: job.model_tier },
     };
 
     this.messageHandler?.(msg);