feat(llm): make tier-C/tier-B model + OpenRouter URL configurable from .env

LLM_MODEL_TIER_C, LLM_MODEL_TIER_B e OPENROUTER_BASE_URL ora override-abili
via env. Default invariati (back-compat). LLMClient accetta i tre valori
come kwargs opzionali; run_phase1 li propaga da Settings.

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

src/multi_swarm/config.py

@@ -26,6 +26,10 @@ 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_b: str = "claude-sonnet-4-6"
+    openrouter_base_url: str = "https://openrouter.ai/api/v1"
+
     run_name: str = "phase1-spike-001"
     data_dir: Path = Field(default=Path("./data"))
     series_dir: Path = Field(default=Path("./series"))