fix: add git remote for proxy so gh CLI can resolve repo context

gh CLI requires a git remote matching GH_HOST to resolve the repo.
With GH_HOST=localhost:18443, gh finds no matching remote and errors:
'none of the git remotes correspond to GH_HOST'

Fix: add a dummy 'proxy' git remote pointing to the proxy host with
the repo path. gh discovers this remote, extracts owner/repo, and
routes API calls through the proxy. The remote is cleaned up in the
stop step.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: lpcox