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/shared/interfaces.ts

@@ -4,6 +4,8 @@ import type {
   LLMProvider,
   McpScope,
   NormalizedMessage,
+  ProviderSkill,
+  ProviderSkillListOptions,
   ProviderAuthStatus,
   ProviderMcpServer,
   UpsertProviderMcpServerInput,
@@ -20,6 +22,7 @@ export interface IProvider {
   readonly id: LLMProvider;
   readonly mcp: IProviderMcp;
   readonly auth: IProviderAuth;
+  readonly skills: IProviderSkills;
   readonly sessions: IProviderSessions;
   readonly sessionSynchronizer: IProviderSessionSynchronizer;
 }
@@ -39,6 +42,22 @@ export interface IProviderAuth {
   getStatus(): Promise<ProviderAuthStatus>;
 }
 
+// ---------------------------
+//----------------- PROVIDER SKILLS INTERFACE ------------
+/**
+ * Skills contract for one provider.
+ *
+ * Implementations discover provider-native skill markdown locations and return
+ * normalized skill records with the exact command syntax expected by that
+ * provider. Each skill is read from a `SKILL.md` file under its skill directory.
+ */
+export interface IProviderSkills {
+  /**
+   * Lists all skills visible to this provider for the optional workspace.
+   */
+  listSkills(options?: ProviderSkillListOptions): Promise<ProviderSkill[]>;
+}
+
 // ---------------------------
 //----------------- PROVIDER MCP INTERFACE ------------
 /**