fix: use fs.realpath instead of manual path.resolve for symlink resolution

resolveSymlink and resolveSymlinkAsync incorrectly resolved relative
symlinks by using path.resolve(filePath, '..', linkTarget). This fails
when symlinks use multi-level relative paths (e.g. ../../skills/...) or
when symlinks are chained (symlink pointing to a directory containing
more symlinks).

Replace with fs.realpathSync/fs.realpath which delegates to the OS for
correct resolution of all symlink types: relative, absolute, chained,
and nested.

Fixes #1738

AI-assisted-by: claude-opus-4.6 via opencode
AI-contribution: partial
AI-session: 20260212-120629-4gTXvDGV

src/shared/file-utils.ts

@@ -1,6 +1,5 @@
-import { lstatSync, readlinkSync } from "fs"
+import { lstatSync, realpathSync } from "fs"
 import { promises as fs } from "fs"
-import { resolve } from "path"
 
 export function isMarkdownFile(entry: { name: string; isFile: () => boolean }): boolean {
   return !entry.name.startsWith(".") && entry.name.endsWith(".md") && entry.isFile()
@@ -16,11 +15,7 @@ export function isSymbolicLink(filePath: string): boolean {
 
 export function resolveSymlink(filePath: string): string {
   try {
-    const stats = lstatSync(filePath, { throwIfNoEntry: false })
-    if (stats?.isSymbolicLink()) {
-      return resolve(filePath, "..", readlinkSync(filePath))
-    }
-    return filePath
+    return realpathSync(filePath)
   } catch {
     return filePath
   }
@@ -28,12 +23,7 @@ export function resolveSymlink(filePath: string): string {
 
 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
+    return await fs.realpath(filePath)
   } catch {
     return filePath
   }