docs: add Gemini OAuth env vars to .env.example [skip-regression-check]

Document GEMINI_MODEL, GEMINI_CREDENTIALS_PATH, GEMINI_API_KEY, and
all extended generation config env vars in the example config file.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

.env.example

@@ -4,7 +4,7 @@ DATABASE_POOL_SIZE=10
 
 # LLM Provider
 # LLM_BACKEND=nearai           # default
-# Possible values: nearai, ollama, openai_compatible, openai, anthropic, github_copilot, tinfoil, openai_codex
+# Possible values: nearai, ollama, openai_compatible, openai, anthropic, github_copilot, tinfoil, openai_codex, gemini_oauth
 # LLM_REQUEST_TIMEOUT_SECS=120  # Increase for local LLMs (Ollama, vLLM, LM Studio)
 
 # === Anthropic Direct ===
@@ -110,6 +110,23 @@ NEARAI_AUTH_URL=https://private.near.ai
 # OPENAI_CODEX_AUTH_URL=https://auth.openai.com  # override (rare)
 # OPENAI_CODEX_API_URL=https://chatgpt.com/backend-api/codex  # override (rare)
 
+# === Google Gemini (OAuth, Gemini CLI compatible) ===
+# LLM_BACKEND=gemini_oauth
+# GEMINI_MODEL=gemini-2.5-flash                  # default
+# GEMINI_CREDENTIALS_PATH=~/.gemini/oauth_creds.json  # default
+# GEMINI_API_KEY=...                             # optional: use API key instead of OAuth
+# GEMINI_API_KEY_AUTH_MECHANISM=query             # "query" (default) or "header"
+# GEMINI_SAFETY_BLOCK_NONE=true                  # disable safety filters (default: false)
+# GEMINI_CLI_CUSTOM_HEADERS=Key:Value,Key2:Value2
+# GEMINI_TOP_P=0.95
+# GEMINI_TOP_K=40
+# GEMINI_SEED=42
+# GEMINI_PRESENCE_PENALTY=0.0
+# GEMINI_FREQUENCY_PENALTY=0.0
+# GEMINI_RESPONSE_MIME_TYPE=application/json
+# GEMINI_RESPONSE_JSON_SCHEMA={"type":"object"}
+# GEMINI_CACHED_CONTENT=cachedContents/abc123
+
 # For full provider setup guide see docs/LLM_PROVIDERS.md
 
 # Channel Configuration