chore: rename placeholder pose '1'; validate README image files are non-empty

The curated pose shipped as '1' with no tags; rename it to a
descriptive name (Laying Back on Elbows) and add browse tags. Also
extend validate_package.py to reject empty README-referenced images,
matching the existing non-empty check for pose previews.

pose_library/local_user_poses/Uncategorized/Laying Back on Elbows.json

@@ -765,6 +765,11 @@
     "asset_type": "pose",
     "repository": "local_user_poses",
     "category": "Uncategorized",
-    "tags": []
+    "tags": [
+      "front view",
+      "laying",
+      "elbows propped",
+      "knees bent"
+    ]
   }
 }

scripts/validate_package.py

@@ -56,7 +56,10 @@ if readme.is_file():
     check(bool(image_paths), "README.md has no embedded showcase images")
     for image_path in image_paths:
         local_path = image_path.split("?", 1)[0]  # strip cache-busting query
-        check((ROOT / local_path).is_file(), f"README image does not exist: {local_path}")
+        target = ROOT / local_path
+        check(target.is_file(), f"README image does not exist: {local_path}")
+        if target.is_file():
+            check(target.stat().st_size > 0, f"empty README image: {local_path}")
 
 for relative in (
     "SKILL.md",