Mirrors the SSHGate/PipelockProxy shape: a host-side prepare that
lifts bottle.git into a tuple of GitGateUpstreams and renders two
shell scripts under stage_dir — the gate's entrypoint (which
initializes a bare repo per upstream and execs git daemon
--enable=receive-pack) and the shared pre-receive hook
(gitleaks-scan, then forward each accepted ref to the real
upstream using the per-repo credential).
Failure in either hook phase aborts the push so the agent sees a
real rejection, not a silent success. KnownHostKey absence is
fail-closed: the hook refuses to forward without a pinned key
rather than TOFU-trusting the upstream from inside the gate.
PRD: docs/prds/0008-git-gate.md
