Users on direct API-key providers (Alibaba, DeepSeek, ZAI, etc.) without
an OpenRouter or Nous key would get broken auxiliary tasks (compression,
vision, etc.) because _resolve_auto() only tried aggregator providers
first, then fell back to iterating PROVIDER_REGISTRY with wrong default
model names.
Now _resolve_auto() checks the user's main provider first. If it's not
an aggregator (OpenRouter/Nous), it uses their main model directly for
all auxiliary tasks. Aggregator users still get the cheap gemini-flash
model as before.
Adds _read_main_provider() to read model.provider from config.yaml,
mirroring the existing _read_main_model().
Reported by SkyLinx — Alibaba Coding Plan user getting 400 errors from
google/gemini-3-flash-preview being sent to DashScope.
93aa01c71c