final mvp created

feat(phase4): critic escalation engine, difficulty tracker, env wiring, gate PASS

feat(phase0): add Groq backend, set as default (llama-3.3-70b-versatile)

feat(phase0): switch default model to Qwen2.5-1.5B-Instruct

refactor(phase0): replace Anthropic SDK with LLMBackend abstraction (default Qwen)