provision_git now does two things: copy the host cwd's .git (when
--cwd is set, existing behavior) and write ~/.gitconfig with
pushInsteadOf rules for each bottle.git entry. A 'git push <real
upstream URL>' from inside the agent transparently rewrites to
'git://<gate>/<name>.git' so the gate gets first crack at the
incoming refs.
pushInsteadOf (not insteadOf) keeps fetch on the original URL —
v1 of the git-gate is push-only scope per PRD 0008. The render
helper is exposed for testing without docker.
