docs: add MARITACA_API_KEY to environment variables template

Add Maritaca AI API key configuration to .env.example for
Carlos Drummond agent's conversational AI capabilities using
Sabiazinho-3 model.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

.env.example

@@ -24,6 +24,7 @@ TRANSPARENCY_API_KEY=your-transparency-api-key-here
 # AI/LLM Providers
 GROQ_API_KEY=your-groq-api-key-here
 OPENAI_API_KEY=your-openai-api-key-here
+MARITACA_API_KEY=your-maritaca-api-key-here
 
 # Vector Database (if using external ChromaDB)
 CHROMADB_HOST=localhost