fix: use utf-8 encoding when reading .env file in load_env()

On Windows, Path.open() defaults to the system ANSI code page (cp1252).
If the .env file contains UTF-8 characters, decoding fails with
'gbk codec can't decode byte 0x94'. Specify encoding='utf-8'
explicitly to ensure consistent behavior across platforms.

tools/skills_tool.py

@@ -111,7 +111,7 @@ def load_env() -> Dict[str, str]:
     if not env_path.exists():
         return env_vars
 
-    with env_path.open() as f:
+    with env_path.open(encoding="utf-8") as f:
         for line in f:
             line = line.strip()
             if line and not line.startswith("#") and "=" in line: