chore: tighten skill manifest validation

Author: jdrhyne

src/skills/schema.ts

@@ -50,11 +50,12 @@ export const validateSkillManifest = (value: unknown): ValidationResult => {
     errors.push("Missing or invalid entry.");
   }
 
-  if (
-    !Array.isArray(record.assets) ||
-    record.assets.some((item) => !isNonEmptyString(item))
-  ) {
-    errors.push("Missing or invalid assets (expected an array of strings).");
+  if (!Array.isArray(record.assets)) {
+    errors.push("Missing assets (expected an array of strings).");
+  } else if (record.assets.length === 0) {
+    errors.push("Assets must include at least one file.");
+  } else if (record.assets.some((item) => !isNonEmptyString(item))) {
+    errors.push("Assets must be non-empty strings.");
   }
 
   if (!isRecord(record.defaults)) {