docs: add all supported API keys to .env.example and AGENTS.md

Author: DeveshParagiri

.env.example

@@ -1,9 +1,20 @@
 # Extropy API Keys
 # These are the only settings that belong in .env (secrets only).
 # For provider/model config, use: extropy config set <key> <value>
+# Set at least one of these depending on which provider you use.
 
-# OpenAI (required if using openai as provider)
+# OpenAI
 OPENAI_API_KEY=sk-...
 
 # Anthropic (from https://console.anthropic.com/settings/keys)
 ANTHROPIC_API_KEY=sk-ant-...
+
+# OpenRouter (from https://openrouter.ai/keys)
+OPENROUTER_API_KEY=sk-or-...
+
+# DeepSeek (from https://platform.deepseek.com/api_keys)
+DEEPSEEK_API_KEY=sk-...
+
+# Azure AI Foundry (AZURE_API_KEY is primary, AZURE_OPENAI_API_KEY is legacy fallback)
+AZURE_API_KEY=...
+# Also requires AZURE_ENDPOINT=https://<resource>.services.ai.azure.com/

AGENTS.md

@@ -136,6 +136,8 @@ export OPENAI_API_KEY=sk-...
 export ANTHROPIC_API_KEY=sk-ant-...
 export OPENROUTER_API_KEY=sk-or-...
 export DEEPSEEK_API_KEY=sk-...
+export AZURE_API_KEY=...
+export AZURE_ENDPOINT=https://<resource>.services.ai.azure.com/
 ```
 
 ## Global Flags