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>

README.md

@@ -296,6 +296,7 @@ automation:
         channel: telegram
         peer: "123456789"
       enabled: false                    # Disabled, won't fire
+      model_tier: fast                  # Use fast tier for quick checks
 ```
 
 ### Cron Config Fields
@@ -309,6 +310,7 @@ automation:
 | `output.peer` | yes | Peer/chat ID on the output channel |
 | `timezone` | no | IANA timezone (defaults to system timezone) |
 | `enabled` | no | Whether the job is active (default: `true`) |
+| `model_tier` | no | Model tier for this job: `fast`, `default`, `complex`, or `local` |
 
 ## Inbound Webhooks