PR 9501 shipped real-home routing for the host system default, and the
env override that could turn it back off was never a shipped control. The
managed-account half of the shared runtime mirror has been unreachable
since: every host account routes to its own self-contained CODEX_HOME
before that code runs.
Delete the flag module and its env plumbing plus the managed branch of
syncForCurrentSelection and the six helpers only it called. The three
lanes that still use the shared mirror -- Windows, a custom CODEX_HOME,
and a hook-lane gate that reports unusable -- are untouched, as are every
legacy migration and the WSL read-back helpers.
4c49989c2e