Each entry pairs a Name (local alias the gate exposes) with an
ssh:// Upstream URL, an IdentityFile the gate uses to push to
that upstream, and an optional KnownHostKey for upstream
host-key pinning. The Upstream URL is parsed at construction
into UpstreamUser/Host/Port/Path so downstream code doesn't
re-parse.
Two cross-validation rules: Names must be unique within a
bottle (each maps to a distinct bare repo), and no git entry's
(host, port) may overlap an ssh entry's (Hostname, Port) — the
same upstream reachable two ways would let a misbehaving agent
route around the gitleaks-bearing git-gate via the L4 ssh-gate.
PRD: docs/prds/0008-git-gate.md
