fix(clean): remove the unprivileged marker with the other proxy state files

Problem

portless clean leaves proxy.unprivileged behind (#529 added the marker but not to the list clean removes), so the next proxy start silently comes back in unprivileged mode.

Changes

• proxy.unprivileged joins the state files clean removes, with the test that covers the list.

Written by an AI agent (Claude) on behalf of @jasonkuhrt.