fix(install): probe /dev/tty by opening it, not bare existence (#16746)

In Docker builds the `/dev/tty` device node is present in the mount
namespace, so `[ -e /dev/tty ]` returns true — but opening it fails
with `ENXIO: No such device or address`. Under the old gate the
"no terminal available" skip never triggered, the setup wizard ran,
and the build aborted a few lines later when bash tried `< /dev/tty`:

    /tmp/install.sh: line 1347: /dev/tty: No such device or address

Replace the existence check with `(: </dev/tty) 2>/dev/null`, which
actually attempts to open /dev/tty in a subshell. The probe succeeds
when piped from `curl | bash` on a real terminal (the wizard's intended
use case) and fails cleanly in Docker build / CI contexts so the skip
kicks in before the redirect can crash.

Add a regression test that statically asserts run_setup_wizard does not
gate on the bare existence check and that the open-based probe is in
place.

Fixes #16746.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/install.sh

@@ -1330,7 +1330,12 @@ run_setup_wizard() {
     # The setup wizard reads from /dev/tty, so it works even when the
     # install script itself is piped (curl | bash). Only skip if no
     # terminal is available at all (e.g. Docker build, CI).
-    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, so the wizard would proceed and
+    # then crash on `< /dev/tty` below.
+    if ! (: </dev/tty) 2>/dev/null; then
         log_info "Setup wizard skipped (no terminal available). Run 'hermes setup' after install."
         return 0
     fi