feat(llm): full multi-tier S/A/B/C/D with routing + pricing

Estende ModelTier a 5 livelli (S/A/B/C/D) con routing automatico:
S/A/B via Anthropic SDK, C/D via OpenRouter (OpenAI SDK). Aggiunge
prezzi per tier S (Opus), A (Sonnet placeholder) e D (Llama). Refactor
LLMClient.complete con dispatch tramite tier_models map e helper
_call_anthropic / _call_openrouter. Settings esposte per tutti e 5
i modelli env-configurabili.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/multi_swarm/config.py

@@ -26,8 +26,11 @@ class Settings(BaseSettings):
     openrouter_api_key: SecretStr
     anthropic_api_key: SecretStr | None = None
 
-    llm_model_tier_c: str = "qwen/qwen-2.5-72b-instruct"
+    llm_model_tier_s: str = "claude-opus-4-7"
+    llm_model_tier_a: str = "claude-sonnet-4-6"
     llm_model_tier_b: str = "claude-sonnet-4-6"
+    llm_model_tier_c: str = "qwen/qwen-2.5-72b-instruct"
+    llm_model_tier_d: str = "meta-llama/llama-3.3-70b-instruct"
     openrouter_base_url: str = "https://openrouter.ai/api/v1"
 
     run_name: str = "phase1-spike-001"