docs: add COACH_PROVIDER to .env.example

.env.example

@@ -11,6 +11,8 @@ XIMILAR_API_KEY=your-ximilar-api-key-here
 PPLX_API_KEY=your-perplexity-api-key-here
 # Perplexity model for deck research & generation (sonar / sonar-pro / sonar-deep-research)
 PPLX_MODEL=sonar-pro
+# Coach chat provider: "local" (Ollama) or "perplexity" (Perplexity API)
+COACH_PROVIDER=local
 
 # CORS allowed origins (comma-separated)
 # Default: http://localhost:5173,http://localhost:8080