refactor(loaders): migrate to async-first pattern for commands and skills

- Remove all sync functions from command loader (async now default)
- Remove sync load functions from skill loader (async now default)
- Add resolveSymlinkAsync to file-utils.ts
- Update all callers to use async versions:
  - config-handler.ts
  - index.ts
  - tools/slashcommand/tools.ts
  - tools/skill/tools.ts
  - hooks/auto-slash-command/executor.ts
  - loader.test.ts
- All 607 tests pass, build succeeds

Generated with assistance of 🤖 [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/index.ts

@@ -34,10 +34,10 @@ import {
 } from "./features/context-injector";
 import { createGoogleAntigravityAuthPlugin } from "./auth/antigravity";
 import {
-  discoverUserClaudeSkillsAsync,
-  discoverProjectClaudeSkillsAsync,
-  discoverOpencodeGlobalSkillsAsync,
-  discoverOpencodeProjectSkillsAsync,
+  discoverUserClaudeSkills,
+  discoverProjectClaudeSkills,
+  discoverOpencodeGlobalSkills,
+  discoverOpencodeProjectSkills,
   mergeSkills,
 } from "./features/opencode-skill-loader";
 import { createBuiltinSkills } from "./features/builtin-skills";
@@ -205,10 +205,10 @@ const OhMyOpenCodePlugin: Plugin = async (ctx) => {
   });
   const includeClaudeSkills = pluginConfig.claude_code?.skills !== false;
   const [userSkills, globalSkills, projectSkills, opencodeProjectSkills] = await Promise.all([
-    includeClaudeSkills ? discoverUserClaudeSkillsAsync() : Promise.resolve([]),
-    discoverOpencodeGlobalSkillsAsync(),
-    includeClaudeSkills ? discoverProjectClaudeSkillsAsync() : Promise.resolve([]),
-    discoverOpencodeProjectSkillsAsync(),
+    includeClaudeSkills ? discoverUserClaudeSkills() : Promise.resolve([]),
+    discoverOpencodeGlobalSkills(),
+    includeClaudeSkills ? discoverProjectClaudeSkills() : Promise.resolve([]),
+    discoverOpencodeProjectSkills(),
   ]);
   const mergedSkills = mergeSkills(
     builtinSkills,