feat(skills): add provider skill management

Users need one settings surface to discover and install skills without manually navigating provider-specific directories.

Add provider-backed global skill installation for Claude, Codex, Gemini, and Cursor, while keeping OpenCode read-only because it reuses other providers' skill locations.

Add a responsive Skills settings tab with scoped discovery, search, refresh controls, markdown and folder uploads, upload feedback, and overflow-safe layouts.

Validate bundled skill files and paths before writing them, preserve scripts and assets, and cover provider discovery and installation behavior with tests.

server/modules/providers/list/codex/codex-skills.provider.ts

@@ -57,4 +57,12 @@ export class CodexSkillsProvider extends SkillsProvider {
 
     return sources;
   }
+
+  protected async getGlobalSkillSource(): Promise<ProviderSkillSource> {
+    return {
+      scope: 'user',
+      rootDir: path.join(os.homedir(), '.agents', 'skills'),
+      commandPrefix: '$',
+    };
+  }
 }