fix(smolmachines): mount git-gate files at boot via virtiofs staging dirs

Two issues caused krun_start_enter -22 (VM boot crash):

1. git-gate entrypoint missing at boot: sidecar_init.py starts all
   daemons (including git-gate) immediately as PID 1. The entrypoint
   was copied in via machine_cp which only runs after machine_start
   returns — too late. virtiofs also can't mount files at root (/).

   Fix: bake a static /git-gate-entrypoint.sh wrapper into the
   Dockerfile.sidecars image that delegates to /etc/git-gate/entrypoint.sh.
   For smolmachines, stage per-bottle scripts with correct in-VM names
   under the git-gate state dir and virtiofs-mount to /etc/git-gate/ at
   VM creation time. docker/macOS backends are unchanged (their file
   bind-mount/docker cp still overrides the static wrapper).

2. Egress confdir mounted read-only: egress_entrypoint.sh writes
   combined-trust.pem to confdir under set -e; mitmproxy also needs
   to write its per-host cert cache there. Both fail fatally on a
   read-only virtiofs mount.

   Fix: change the egress confdir virtiofs mount to writable.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile.sidecars

@@ -14,9 +14,10 @@
 #   /app/supervise_server.py + .py       supervise MCP server
 #   /app/sidecar_init.py                 PID 1 supervisor
 #   /etc/egress/routes.yaml              bind-mounted at run time
-#   /etc/git-gate/pre-receive            docker-cp'd at start time
-#   /git-gate-entrypoint.sh              docker-cp'd at start time
-#   /git-gate/creds/*                    docker-cp'd at start time
+#   /etc/git-gate/entrypoint.sh          per-bottle (docker-cp or virtiofs mount)
+#   /etc/git-gate/pre-receive            per-bottle (docker-cp or virtiofs mount)
+#   /git-gate-entrypoint.sh              static wrapper → /etc/git-gate/entrypoint.sh
+#   /git-gate/creds/*                    per-bottle (docker-cp or virtiofs mount)
 #   /git/*                               bare repos, populated at runtime
 #   /run/supervise/bot-bottle.db         bind-mounted at run time
 #   /home/mitmproxy/.mitmproxy/          mitmproxy CA dir
@@ -90,6 +91,17 @@ RUN mkdir -p \
       /run/supervise \
       /home/mitmproxy/.mitmproxy
 
+# Static wrapper for the git-gate entrypoint. The per-bottle
+# entrypoint script is either:
+#   - docker-cp'd to /git-gate-entrypoint.sh (docker/macOS backends),
+#     which overwrites this wrapper; or
+#   - virtiofs-mounted at /etc/git-gate/entrypoint.sh (smolmachines),
+#     where this wrapper delegates to it at runtime.
+# Either way sidecar_init.py calls `/bin/sh /git-gate-entrypoint.sh`.
+RUN printf '#!/bin/sh\nexec /etc/git-gate/entrypoint.sh "$@"\n' \
+      > /git-gate-entrypoint.sh \
+    && chmod 755 /git-gate-entrypoint.sh
+
 # Documentation only — the compose renderer publishes whichever
 # subset the bottle uses.
 EXPOSE 8888 9099 9418 9420 9100