auth(cursor): honor JCODE_HOME for auth.json path on Linux

The Linux branch of cursor_auth_file_path() used dirs::config_dir()
directly, bypassing the JCODE_HOME redirect that every other external-CLI
auth detector (Codex, Claude, Gemini, Copilot) respects. In the onboarding
sandbox (which sets JCODE_HOME to an empty dir), this leaked the real
~/.config/cursor/auth.json so only Cursor appeared as importable while
Codex/Anthropic/etc. correctly found nothing. Now Cursor resolves to
$JCODE_HOME/external/.config/cursor/auth.json like the others.