fix(plugin-loader): preserve scoped npm package names in plugin key parsing

Scoped packages like @scope/pkg were truncated to just 'pkg' because
basename() strips the scope prefix. Fix:
- Detect scoped packages (starting with @) and find version separator
  after the scope slash, not at the leading @
- Return full scoped name (@scope/pkg) instead of calling basename
- Add regression test for scoped package name preservation

src/features/claude-code-plugin-loader/discovery.ts

@@ -81,7 +81,14 @@ function loadPluginManifest(installPath: string): PluginManifest | null {
 
 function derivePluginNameFromKey(pluginKey: string): string {
   const keyWithoutSource = pluginKey.startsWith("npm:") ? pluginKey.slice(4) : pluginKey
-  const versionSeparator = keyWithoutSource.lastIndexOf("@")
+
+  let versionSeparator: number
+  if (keyWithoutSource.startsWith("@")) {
+    const scopeEnd = keyWithoutSource.indexOf("/")
+    versionSeparator = scopeEnd > 0 ? keyWithoutSource.indexOf("@", scopeEnd) : -1
+  } else {
+    versionSeparator = keyWithoutSource.lastIndexOf("@")
+  }
   const keyWithoutVersion = versionSeparator > 0 ? keyWithoutSource.slice(0, versionSeparator) : keyWithoutSource
 
   if (keyWithoutVersion.startsWith("file://")) {
@@ -92,6 +99,10 @@ function derivePluginNameFromKey(pluginKey: string): string {
     }
   }
 
+  if (keyWithoutVersion.startsWith("@") && keyWithoutVersion.includes("/")) {
+    return keyWithoutVersion
+  }
+
   if (keyWithoutVersion.includes("/") || keyWithoutVersion.includes("\\")) {
     return basename(keyWithoutVersion)
   }