fix: prove Linux Agent sandbox boundary

hub/deploy/README.md

@@ -1,8 +1,8 @@
 # Hub service installation
 
 The initial production target is a supported Linux host with systemd,
-PostgreSQL, Node.js 20 or newer, `pg_isready`, `runuser`, `setpriv`, bubblewrap
-and `cph`. The Hub runs as the dedicated non-login `cph-hub` user; do not
+PostgreSQL, Node.js 20 or newer, `pg_isready`, `runuser`, `setpriv`, bubblewrap,
+`socat` and `cph`. The Hub runs as the dedicated non-login `cph-hub` user; do not
 create or run the unit as root.
 
 Build or deploy the Hub under `/srv/curriculum-project-hub`, then run:
@@ -18,13 +18,13 @@ On a clean host the first invocation creates
 production key and exits with status 78. Fill every required blank, set
 `CPH_BIN` to an executable compatible `cph`, keep the file mode at `0600`, and
 run the same command again. No systemd unit is installed until configuration,
-paths, Node, bubblewrap, `cph --version`, PostgreSQL connectivity and directory
+paths, Node, bubblewrap, `socat`, `cph --version`, PostgreSQL connectivity and directory
 ownership all pass preflight.
 
 An existing service account is accepted only when its primary group, home and
 non-login shell match the requested configuration and it has no supplementary
 groups. After provisioning, the installer executes path-access, built Hub,
-Prisma, an authenticated database query, `cph` and bubblewrap namespace probes
+Prisma, an authenticated database query, `cph`, `socat` and bubblewrap namespace probes
 as that account with `no_new_privs` set. Inaccessible parents, UID-specific
 database/network policy, or a host that forbids unprivileged user namespaces
 therefore fail before unit installation.