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.

hermes_cli/config.py

@@ -159,7 +159,7 @@ DEFAULT_CONFIG = {
     "compression": {
         "enabled": True,
         "threshold": 0.50,
-        "summary_model": "google/gemini-3-flash-preview",
+        "summary_model": "",  # empty = use main configured model
         "summary_provider": "auto",
         "summary_base_url": None,
     },
@@ -1659,7 +1659,8 @@ def show_config():
     print(f"  Enabled:      {'yes' if enabled else 'no'}")
     if enabled:
         print(f"  Threshold:    {compression.get('threshold', 0.50) * 100:.0f}%")
-        print(f"  Model:        {compression.get('summary_model', 'google/gemini-3-flash-preview')}")
+        _sm = compression.get('summary_model', '') or '(main model)'
+        print(f"  Model:        {_sm}")
         comp_provider = compression.get('summary_provider', 'auto')
         if comp_provider != 'auto':
             print(f"  Provider:     {comp_provider}")