fix(install): widen /dev/tty open-probe to sibling gates (#16746)

The contributor's PR (#16750) scoped the fix to run_setup_wizard() and
explicitly punted the two sibling sites. Both have the identical
[ -e /dev/tty ] pattern followed by a < /dev/tty redirect and crash in
Docker the same way:

- scripts/install.sh:732 install_system_packages() -- apt sudo prompt
  fallback. sudo ... < /dev/tty dies with the same ENXIO.
- scripts/install.sh:1395 maybe_start_gateway() -- gateway-install gate,
  same function path as the wizard reproducer.

Fix both with the same (: </dev/tty) 2>/dev/null probe, and parametrize
the regression test over all three gated functions so any future
regression is caught regardless of which site breaks.

scripts/install.sh

@@ -729,9 +729,12 @@ install_system_packages() {
                         return 0
                     fi
                 fi
-            elif [ -e /dev/tty ]; then
+            elif (: </dev/tty) 2>/dev/null; then
                 # Non-interactive (e.g. curl | bash) but a terminal is available.
                 # Read the prompt from /dev/tty (same approach the setup wizard uses).
+                # Probe by actually opening /dev/tty: a bare existence test passes
+                # in Docker builds where the device node is in the mount namespace
+                # but opening fails with ENXIO. See #16746.
                 echo ""
                 log_info "sudo is needed ONLY to install optional system packages (${pkgs[*]}) via your package manager."
                 log_info "Hermes Agent itself does not require or retain root access."
@@ -1397,7 +1400,10 @@ maybe_start_gateway() {
         fi
     fi
 
-    if ! [ -e /dev/tty ]; then
+    # Probe by actually opening /dev/tty: a bare existence test passes
+    # in Docker builds where the device node is in the mount namespace
+    # but opening fails with ENXIO. See #16746.
+    if ! (: </dev/tty) 2>/dev/null; then
         log_info "Gateway setup skipped (no terminal available). Run 'hermes gateway install' later."
         return 0
     fi