fix: return resolved path from validatePath to prevent symlink write bypass

Previously, validatePath() resolved symlinks for the allowlist check but
returned the original (unresolved) path. This meant callers like writeFile()
and createDirectory() would still operate on the symlink path, following it
to the restricted target. Now validatePath() consistently returns the
resolved (canonical) path so all subsequent file operations use the real
target path, closing the symlink write bypass vector.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: sorlen008

src/tools/filesystem.ts

@@ -246,25 +246,25 @@ export async function validatePath(requestedPath: string): Promise<string> {
             throw new Error(`Path not allowed: ${requestedPath}. Must be within one of these directories: ${(await getAllowedDirs()).join(', ')}`);
         }
 
+        // SECURITY: Always return the resolved path (with symlinks resolved) so that
+        // all subsequent file operations (read, write, mkdir, etc.) operate on the
+        // canonical target, not on a symlink that could point outside allowed directories.
+        // pathForNextCheck already holds resolvedRealPath ?? absoluteOriginal from above.
+
         // Check if path exists
         try {
             // fs.stat() will automatically follow symlinks, so we get existence info
-            const stats = await fs.stat(absoluteOriginal);
-            // If path exists, resolve any symlinks
-            if (resolvedRealPath) {
-                return resolvedRealPath;
-            }
-
-            return absoluteOriginal;
+            await fs.stat(pathForNextCheck);
+            return pathForNextCheck;
         } catch (error) {
             // Path doesn't exist - validate parent directories
-            if (await validateParentDirectories(absoluteOriginal)) {
-                // Return the path if a valid parent exists
+            if (await validateParentDirectories(pathForNextCheck)) {
+                // Return the resolved path if a valid parent exists
                 // This will be used for folder creation and many other file operations
-                return absoluteOriginal;
+                return pathForNextCheck;
             }
-            // If no valid parent found, return the absolute path anyway
-            return absoluteOriginal;
+            // If no valid parent found, return the resolved path anyway
+            return pathForNextCheck;
         }
     };