Step 1 of _resolve_auto() explicitly excluded 'custom' providers,
forcing custom endpoint users through the fragile fallback chain
instead of using their known-working main model credentials.
This caused silent compression failures for users on local OpenAI-
compatible endpoints — the summary generation would fail, middle
turns would be silently dropped, and the agent would lose all
conversation context.
Remove 'custom' from the exclusion list so custom endpoint users
get the same main-model-first treatment as DeepSeek, Anthropic,
Gemini, and other direct providers.