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>

.env.example

@@ -8,6 +8,11 @@ CERBERO_BOT_TAG=swarm-poc-phase1
 OPENROUTER_API_KEY=
 ANTHROPIC_API_KEY=
 
+# LLM models (override Phase 1 defaults if needed)
+LLM_MODEL_TIER_C=qwen/qwen-2.5-72b-instruct
+LLM_MODEL_TIER_B=claude-sonnet-4-6
+OPENROUTER_BASE_URL=https://openrouter.ai/api/v1
+
 # Run config
 RUN_NAME=phase1-spike-001
 DATA_DIR=./data