Add LiteLLM passthrough adapter for proxy

[endre82]

Allow Meridian proxy to detect and handle LiteLLM-style requests via x-litellm-* headers.
When LiteLLM is configured as a proxy provider, it sends x-litellm-* headers with requests. This change adds detection for these headers and routes such requests to a passthrough adapter that forwards them directly to Claude Code SDK without session management or tool blocking.

• Added hasLiteLLMHeaders() detection in detect.ts
• Created passthroughAdapter in adapters/passthrough.ts
• Added DEBUG_PROXY env var for cache hit/miss visibility in cache.ts
• Added unit tests for passthrough detection

[endre82]

I know, this proxy is focusing on OpenCode, but I would like to extend the usage to LiteLLM.

I've a fix developed for that already, I would like to commit to a branch for review.