fix(compression): remove hardcoded gemini-3-flash-preview as default summary model

Closes #2453

The DEFAULT_CONFIG was hardcoding google/gemini-3-flash-preview as the
summary_model for context compression. This caused unexpected OpenRouter
charges for users who configured a different provider/model, because the
compression task would silently fall back to gemini via OpenRouter even
when the user's main model was on a different provider.

Fix: change summary_model default to empty string. When empty,
call_llm() resolves the model through the standard auto-detection chain
(auxiliary.compression config -> env vars -> main provider), which
correctly uses the user's configured provider and model.

Users who want a dedicated cheap model for compression can still
explicitly set compression.summary_model in their config.yaml.