feat: add Kilo Code (kilocode) as first-class inference provider (#1666)

Add Kilo Gateway (kilo.ai) as an API-key provider with OpenAI-compatible
endpoint at https://api.kilo.ai/api/gateway. Supports 500+ models from
Anthropic, OpenAI, Google, xAI, Mistral, MiniMax via a single API key.

- Register kilocode in PROVIDER_REGISTRY with aliases (kilo, kilo-code,
  kilo-gateway) and KILOCODE_API_KEY / KILOCODE_BASE_URL env vars
- Add to model catalog, CLI provider menu, setup wizard, doctor checks
- Add google/gemini-3-flash-preview as default aux model
- 12 new tests covering registration, aliases, credential resolution,
  runtime config
- Documentation updates (env vars, config, fallback providers)
- Fix setup test index shift from provider insertion

Inspired by PR #1473 by @amanning3390.

Co-authored-by: amanning3390 <amanning3390@users.noreply.github.com>

agent/auxiliary_client.py

@@ -60,6 +60,7 @@ _API_KEY_PROVIDER_AUX_MODELS: Dict[str, str] = {
     "ai-gateway": "google/gemini-3-flash",
     "opencode-zen": "gemini-3-flash",
     "opencode-go": "glm-5",
+    "kilocode": "google/gemini-3-flash-preview",
 }
 
 # OpenRouter app attribution headers