feat(litellm): front Kimi K3 (Moonshot) as a paid gateway passthrough

Adds model_name kimi-k3 → openai/kimi-k3 @ https://api.moonshot.ai/v1
(OpenAI-compatible), keyed by MOONSHOT_API_KEY (compose env + .env.example
placeholder; real key on server only). Verified live through the gateway.

Two Moonshot constraints captured in the config comment + pinned: K3 accepts
ONLY temperature=1 (else 400), and it is a reasoning model (CoT in
reasoning_content, answer in content — needs adequate max_tokens or content
returns empty). Model id confirmed via /v1/models.

stacks/litellm/.env.example

@@ -35,6 +35,9 @@ VLLM_API_KEY=
 # (glm-5.1 / glm-5-turbo / glm-4.7 / glm-4.5-air).
 Z_AI_API_KEY=
 
+# Moonshot AI (Kimi) key — fronts kimi-k3 (https://api.moonshot.ai/v1). PAID.
+MOONSHOT_API_KEY=
+
 # --- Langfuse-ready (leave blank for the lean first cut) ---
 # Fill these AND uncomment success/failure_callback in conf/config.yaml
 # to ship full traces to a Langfuse instance. That is the whole upgrade.