fix: remove AirLLM config settings from config.py

Remove `airllm` from timmy_model_backend Literal type and delete the
airllm_model_size field plus associated comments. Replace the one
settings.airllm_model_size reference in agent.py with a hardcoded
default, and clean up mock assignments in tests.

Fixes #473

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/infrastructure/router/cascade.py

@@ -826,7 +826,9 @@ class CascadeRouter:
             Summary dict with added/removed/preserved counts.
         """
         # Snapshot current runtime state keyed by provider name
-        old_state: dict[str, tuple[ProviderMetrics, CircuitState, float | None, int, ProviderStatus]] = {}
+        old_state: dict[
+            str, tuple[ProviderMetrics, CircuitState, float | None, int, ProviderStatus]
+        ] = {}
         for p in self.providers:
             old_state[p.name] = (
                 p.metrics,