fix(model-resolution): honor user config overrides on cold cache

When provider-models cache is cold (first run / cache miss),
resolveModelForDelegateTask returns {skipped: true}. Previously this
caused the subagent resolver to:

1. Ignore the user's explicit model override (e.g. explore.model)
2. Fall through to the hardcoded fallback chain which may contain
   model IDs that don't exist in the provider catalog

Now:
- subagent-resolver: if resolution is skipped but user explicitly
  configured a model, use it directly
- subagent-resolver: don't assign hardcoded fallback chain on skip
- category-resolver: same — don't leak hardcoded chain on skip
- general-agents: if user model fails resolution, use it as-is
  instead of falling back to hardcoded chain first entry

Closes #2820

src/tools/delegate-task/category-resolver.ts

@@ -239,6 +239,7 @@ Available categories: ${categoryNames.join(", ")}`,
     modelInfo,
     actualModel,
     isUnstableAgent,
-    fallbackChain: configuredFallbackChain ?? requirement?.fallbackChain,
+    // Don't use hardcoded fallback chain when resolution was skipped (cold cache)
+    fallbackChain: configuredFallbackChain ?? (isModelResolutionSkipped ? undefined : requirement?.fallbackChain),
   }
 }