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/shared/file-utils.ts

@@ -1,4 +1,5 @@
 import { lstatSync, readlinkSync } from "fs"
+import { promises as fs } from "fs"
 import { resolve } from "path"
 
 export function isMarkdownFile(entry: { name: string; isFile: () => boolean }): boolean {
@@ -24,3 +25,16 @@ export function resolveSymlink(filePath: string): string {
     return filePath
   }
 }
+
+export async function resolveSymlinkAsync(filePath: string): Promise<string> {
+  try {
+    const stats = await fs.lstat(filePath)
+    if (stats.isSymbolicLink()) {
+      const linkTarget = await fs.readlink(filePath)
+      return resolve(filePath, "..", linkTarget)
+    }
+    return filePath
+  } catch {
+    return filePath
+  }
+}