fix: model format normalization and explicit config cache bypass

- Add normalizeModelFormat() utility for string/object model handling
- Update subagent-resolver to handle both model formats
- Add explicitUserConfig flag to ModelResolutionResult
- Set explicitUserConfig: true when user model is found in pipeline

This fixes the issue where plugin-provided models fail cache validation
and fall through to random fallback models.

src/shared/model-format-normalizer.ts

@@ -0,0 +1,20 @@
+export function normalizeModelFormat(
+  model: string | { providerID: string; modelID: string }
+): { providerID: string; modelID: string } | undefined {
+  if (!model) {
+    return undefined
+  }
+
+  if (typeof model === "object" && "providerID" in model && "modelID" in model) {
+    return { providerID: model.providerID, modelID: model.modelID }
+  }
+
+  if (typeof model === "string") {
+    const parts = model.split("/")
+    if (parts.length >= 2) {
+      return { providerID: parts[0], modelID: parts.slice(1).join("/") }
+    }
+  }
+
+  return undefined
+}