feat(providers): surface skills in slash command menu

Provider skills were hidden behind provider-specific filesystem rules.

That made the backend and UI unable to offer one discovery path for skills.

Add a normalized skills contract, provider service, and provider skills API.

Keep provider-specific lookup rules inside adapters so routes and UI stay generic.

Claude needs plugin handling because enabled plugins resolve through installed_plugins.json.

Plugin folders can expose commands or skills, so Claude scans both forms.

Claude plugin commands are namespaced to avoid collisions with user and project skills.

Codex, Gemini, and Cursor adapters map their expected skill roots into the same contract.

The slash menu now shows skills beside built-in and custom commands for discovery.

The menu avoids mid-message activation, duplicate rows, loose namespace matches, and input overlap.

Provider tests cover discovery locations and Claude plugin edge cases.

server/modules/providers/shared/base/abstract.provider.ts

@@ -3,6 +3,7 @@ import type {
   IProviderAuth,
   IProviderMcp,
   IProviderSessionSynchronizer,
+  IProviderSkills,
   IProviderSessions,
 } from '@/shared/interfaces.js';
 import type { LLMProvider } from '@/shared/types.js';
@@ -18,6 +19,7 @@ export abstract class AbstractProvider implements IProvider {
   readonly id: LLMProvider;
   abstract readonly mcp: IProviderMcp;
   abstract readonly auth: IProviderAuth;
+  abstract readonly skills: IProviderSkills;
   abstract readonly sessions: IProviderSessions;
   abstract readonly sessionSynchronizer: IProviderSessionSynchronizer;