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.test.ts

@@ -121,6 +121,32 @@ describe('CronScheduler', () => {
     warnSpy.mockRestore();
   });
 
+  it('triggerJob includes model_tier in metadata when configured', () => {
+    const jobs = [makeCronJob({ model_tier: 'fast' })];
+    scheduler = new CronScheduler(jobs, mockChannelRegistry as any);
+
+    const messages: InboundMessage[] = [];
+    scheduler.onMessage((msg: InboundMessage) => messages.push(msg));
+
+    scheduler.triggerJob('test-job');
+
+    expect(messages).toHaveLength(1);
+    expect(messages[0].metadata?.modelTier).toBe('fast');
+  });
+
+  it('triggerJob metadata.modelTier is undefined when not configured', () => {
+    const jobs = [makeCronJob()];
+    scheduler = new CronScheduler(jobs, mockChannelRegistry as any);
+
+    const messages: InboundMessage[] = [];
+    scheduler.onMessage((msg: InboundMessage) => messages.push(msg));
+
+    scheduler.triggerJob('test-job');
+
+    expect(messages).toHaveLength(1);
+    expect(messages[0].metadata?.modelTier).toBeUndefined();
+  });
+
   it('lists registered job names', () => {
     const jobs = [
       makeCronJob({ name: 'job-a' }),