fix(installer): stop skill scanner from recursing into discovered skills

Skills don't nest. Once the manifest generator finds a valid SKILL.md
in a directory, it should not recurse into that skill's subdirectories
looking for more skills. Template files (like bmb's setup-skill-template)
inside a skill's assets/ would be incorrectly scanned and produce
spurious errors.

tools/installer/core/manifest-generator.js

@@ -193,12 +193,14 @@ class ManifestGenerator {
           }
         }
 
-        // Recurse into subdirectories
+        // Recurse into subdirectories — but not inside a discovered skill
+        if (!skillMeta) {
           for (const entry of entries) {
             if (!entry.isDirectory()) continue;
             if (entry.name.startsWith('.') || entry.name.startsWith('_')) continue;
             await walk(path.join(dir, entry.name));
           }
+        }
       };
 
       await walk(modulePath);