chore(cloud): add CEREBRAS_API_KEY to .env.example

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: lalalune

cloud/.env.example

@@ -127,6 +127,10 @@ ANTHROPIC_API_KEY=sk-ant-your_anthropic_key_here
 # Get from: https://console.groq.com/keys
 GROQ_API_KEY=your_groq_api_key_here
 
+# Cerebras API Key (ultra-fast inference for provisioning placeholder agent)
+# Get from: https://cloud.cerebras.ai/platform/apikeys
+CEREBRAS_API_KEY=your_cerebras_api_key_here
+
 # Fal.ai API Key (for video generation)
 # Get from: https://fal.ai/dashboard/keys
 # Format: "key_id:key_secret"