Background subagents (explore/librarian) failed with auth errors because
resolveModelForDelegateTask() always picked the first fallback entry when
availableModels was empty — often an unauthenticated provider like xai
or opencode-go.
Fix: when connectedProvidersCache is populated, iterate fallback chain
and pick the first entry whose provider is in the connected set.
Legacy behavior preserved when cache is null (not yet populated).
- model-selection.ts: use readConnectedProvidersCache to filter fallback chain
- test: 4 new tests for connected-provider-aware resolution
Before provider cache exists (first run), resolveModelForDelegateTask now
returns undefined instead of guessing a model. This lets OpenCode use its
system default model when no model is specified in the prompt body.
User-specified model overrides still take priority regardless of cache state.
YeonGyu-Kim