fix(cli): decode .env as UTF-8 to avoid GBK crash on Windows

Path.read_text() uses the system locale by default. On Windows CN/JP/KR
locales (GBK/CP932/CP949), reading a UTF-8 .env raises UnicodeDecodeError
as soon as it contains any non-ASCII byte (e.g. an em dash).

Pin encoding="utf-8" on every .env read in hermes_cli to match how the
rest of the codebase (load_dotenv at doctor.py:26) already decodes it.

Adds a regression test that monkeypatches Path.read_text to simulate a
GBK locale and asserts 'hermes doctor' no longer raises.

Refs #18637

hermes_cli/memory_setup.py

@@ -361,7 +361,7 @@ def _write_env_vars(env_path: Path, env_writes: dict) -> None:
 
     existing_lines = []
     if env_path.exists():
-        existing_lines = env_path.read_text().splitlines()
+        existing_lines = env_path.read_text(encoding="utf-8").splitlines()
 
     updated_keys = set()
     new_lines = []