chore: add .env.example with all required environment variables

.env.example

@@ -0,0 +1,19 @@
+# Copy this to .env and fill in your values
+# Never commit .env to git
+
+# HuggingFace
+HF_TOKEN=hf_your_token_here
+API_BASE_URL=https://api-inference.huggingface.co/models
+MODEL_NAME=mistralai/Mistral-7B-Instruct-v0.3
+
+# Anthropic (used by P3's attack classifier as fallback)
+ANTHROPIC_API_KEY=sk-ant-your_key_here
+
+# Server
+MAX_TURNS=10
+DEBUG=false
+DEFAULT_LLM_PROVIDER=huggingface
+
+# Timeouts
+LLM_TIMEOUT=30
+LLM_MAX_RETRIES=3