offer to add an ssh key even when one already exists

The zip option was already gone — it never came across in the port, since the
file is key material that cannot live in the repository and the script no longer
sits next to it. Pasting a public key was already the first option. What was
missing is the case where the account HAS a key: the section went straight to
hardening, so there was no way to authorise a second machine, a rebuilt laptop or
anyone else, and the original had no way to do it at all.

The same menu is now offered either way. What differs is whether it can be
declined without consequence: with no key, declining means the hardening below
refuses too, and the run says so rather than quietly moving on.

confirm() takes an optional default so this one can be [y/N]. Most questions in
this script are "do the thing you already asked for" and Enter should mean yes; a
genuine extra defaulting to yes is how people end up agreeing to things by
reflex.

A pasted key is trimmed before validation. Copying from a terminal or a password
manager routinely brings leading or trailing whitespace, and ssh-keygen will not
parse a key with it attached — which would have read as "that is not a valid
key" for a key that is perfectly fine.

Also corrected the reason unzip is in core utils, which still said it was there
to open ssh-keys.zip.

Verified: the add-another prompt appears and defaults to no, a whitespace-wrapped
key is trimmed and accepted, and the already-hardened path is unchanged.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

scripts/setup/machine-setup/lib/base.sh

@@ -145,18 +145,26 @@ prompt_value() {
 #
 # ASSUME_YES=1 answers all of them, for an unattended run.
 confirm() {
-  local message="${1:-Proceed?}" answer
-  [[ "${ASSUME_YES:-}" == "1" ]] && return 0
+  local message="${1:-Proceed?}"
+  # Second argument flips the default. Most questions here are "do the thing you
+  # already asked for" and Enter should mean yes; a few are genuine extras, where
+  # defaulting to yes would have people agreeing to them by reflex.
+  local default="${2:-y}" answer prompt
+
+  [[ "${ASSUME_YES:-}" == "1" ]] && { [[ "$default" == "y" ]] && return 0 || return 1; }
+
+  if [[ "$default" == "y" ]]; then prompt="[Y/n]"; else prompt="[y/N]"; fi
 
   while true; do
     # EOF is not a yes. Without this an unattended run without ASSUME_YES would
     # spin here forever.
-    if ! read -rp "  ${message} [Y/n]: " answer; then
+    if ! read -rp "  ${message} ${prompt}: " answer; then
       echo ""
       fail "No answer. Set ASSUME_YES=1 to run without prompts."
     fi
+    [[ -z "$answer" ]] && answer="$default"
     case "$answer" in
-      "" | y | Y | yes | Yes) return 0 ;;
+      y | Y | yes | Yes) return 0 ;;
       n | N | no | No) return 1 ;;
       *) warn "Answer y or n." ;;
     esac