fix(bootstrap): detect a missing terminal by opening /dev/tty

prompt_install_mode guarded its prompt with `[ ! -r /dev/tty ]`, which
never fires on Linux: /dev/tty is mode 0666 whether or not the process
has a controlling terminal, and only opening it fails. Without a
terminal the menu was printed, the read failed with "No such device or
address", and the default was taken by accident rather than by the
documented path.

The guard now opens /dev/tty in a subshell and takes the "no terminal
for a prompt" path when that fails.

deploy/bootstrap.sh

@@ -2240,8 +2240,9 @@ prompt_install_mode() {
 
   # No override: ask on the controlling terminal. Under `curl | sudo bash` stdin
   # is the script, so we must read /dev/tty, not stdin. No tty (CI/cloud-init) →
-  # take the default.
-  if [ ! -r /dev/tty ]; then
+  # take the default. Open it to find out: /dev/tty is mode 0666 on every Linux host, so
+  # `-r` passes even when there is no controlling terminal and only the open fails.
+  if ! (: </dev/tty) 2>/dev/null; then
     INSTALL_MODE="$def"
     log "no terminal for a prompt; defaulting to a ${def} install (set FELIS_INSTALL_MODE=full or nano to override)"
     return 0