refactor(workspace): extract getChatOnlyEnvOverrides, add Windows APPDATA isolation

- Move chat-only env overrides to getChatOnlyEnvOverrides() in workspace.ts
- On Windows: set APPDATA and LOCALAPPDATA to workspace subdirs
- On Unix: set XDG_CONFIG_HOME to workspace/.config
- Create AppData/Roaming, AppData/Local on Windows temp workspace
- Create .cursor/rules and .config dirs for isolated agent