feat(quick-260327-x64): implement listSkills/syncSkills on Claude, Codex, and Gemini adapters

- Add skills.ts to Claude adapter: ephemeral mode, listClaudeSkills/syncClaudeSkills
- Add skills.ts to Codex adapter: ephemeral mode, listCodexSkills/syncCodexSkills
- Add skills.ts to Gemini adapter: persistent mode with symlink sync, listGeminiSkills/syncGeminiSkills
- Re-export skill functions from each adapter's server/index.ts

packages/adapter-gemini-local/src/server/index.ts

@@ -7,6 +7,7 @@ export {
   detectGeminiAuthRequired,
   isGeminiTurnLimitResult,
 } from "./parse.js";
+export { listGeminiSkills, syncGeminiSkills } from "./skills.js";
 import type { AdapterSessionCodec } from "@agentcompany/adapter-utils";
 
 function readNonEmptyString(value: unknown): string | null {