simplify WORKSPACE to a hardcoded constant

WORKSPACE_DIR env var was never set in docker-compose.yml, so the
default always applied anyway. The container-internal mount point
is always /workspace, so there's no reason to make it configurable.

server/main.py

@@ -18,7 +18,7 @@ from typing import Any
 logging.basicConfig(level=logging.INFO, format="%(asctime)s [%(levelname)s] %(message)s")
 log = logging.getLogger("mcp")
 
-WORKSPACE = os.environ.get("WORKSPACE_DIR", "/workspace")
+WORKSPACE = "/workspace"
 
 import pyte
 import ptyprocess