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

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),
   }
 }

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

@@ -125,12 +125,26 @@ Create the work plan directly - that's your job as the planning agent.`,
         systemDefaultModel: undefined,
       })
 
-      if (resolution && !('skipped' in resolution)) {
+      const resolutionSkipped = resolution && 'skipped' in resolution
+
+      if (resolution && !resolutionSkipped) {
         const normalized = normalizeModelFormat(resolution.model)
         if (normalized) {
           const variantToUse = agentOverride?.variant ?? resolution.variant
           categoryModel = variantToUse ? { ...normalized, variant: variantToUse } : normalized
         }
+      } else if (resolutionSkipped && agentOverride?.model) {
+        // Cold cache: resolution was skipped but user explicitly configured a model.
+        // Honor the user override directly — don't fall through to hardcoded fallback chain.
+        const normalized = normalizeModelFormat(agentOverride.model)
+        if (normalized) {
+          const variantToUse = agentOverride?.variant
+          categoryModel = variantToUse ? { ...normalized, variant: variantToUse } : normalized
+          log("[delegate-task] Cold cache: using explicit user override for subagent", {
+            agent: agentToUse,
+            model: agentOverride.model,
+          })
+        }
       }
 
       const defaultProviderID = categoryModel?.providerID
@@ -140,7 +154,9 @@ Create the work plan directly - that's your job as the planning agent.`,
         normalizedAgentFallbackModels,
         defaultProviderID,
       )
-      fallbackChain = configuredFallbackChain ?? agentRequirement?.fallbackChain
+      // Don't assign hardcoded fallback chain when resolution was skipped (cold cache)
+      // — the chain may contain model IDs that don't exist in the provider yet.
+      fallbackChain = configuredFallbackChain ?? (resolutionSkipped ? undefined : agentRequirement?.fallbackChain)
 
       // Only promote fallback-only settings when resolution actually selected a fallback model.
       const resolvedFallbackEntry = (resolution && !('skipped' in resolution)) ? resolution.fallbackEntry : undefined