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/agents/builtin-agents/general-agents.ts

@@ -78,12 +78,16 @@ export function collectPendingBuiltinAgents(input: {
     })
     if (!resolution) {
       if (override?.model) {
-        log("[agent-registration] User-configured model could not be resolved, falling back", {
+        // User explicitly configured a model but resolution failed (e.g., cold cache).
+        // Honor the user's choice directly instead of falling back to hardcoded chain.
+        log("[agent-registration] User-configured model not resolved, using as-is", {
           agent: agentName,
           configuredModel: override.model,
         })
+        resolution = { model: override.model, provenance: "override" as const }
+      } else {
+        resolution = getFirstFallbackModel(requirement)
       }
-      resolution = getFirstFallbackModel(requirement)
     }
     if (!resolution) continue
     const { model, variant: resolvedVariant } = resolution