Wire agent-scoped solo push gate for solo-mars-cc (.solo-audit.sh pattern)

EVO-50 protocol point 5: lane separation mechanically enforced via agent-scoped push gate. Port venus .solo-audit.sh content-hash artifact gate (scoped LLMMSG_AGENT==solo-mars-cc so coder-lane pushes never hit it and vice versa; venus VENUS-112 / ayudarg AYUD-112 pattern) into the Mars repo push path. Until wired, solo-mars-cc's always-audit (spawned one-shot Sonnet auditor -> PASS/BLOCK -> content-hash-gated push) is discipline-only. solo-mars-cc offered to help wire it. Coordinate with .gitpush-pre.sh (strict --only mode) so the gates compose.