default the username to whoever ran sudo, and look up their real home

Three changes to the question at the top of the run.

USER_HOME is looked up rather than assumed. The original built "/home/$USERNAME",
which is only the usual answer — an account created with a different home, or one
whose home was moved, had every later step writing to a directory that was not
theirs. getent passwd knows; the /home guess remains only as the fallback for an
account that does not exist yet, where there is nothing to look up.

The default is now whoever invoked sudo. On a re-run, or on a machine that is
already somebody's, that is the answer every time, and retyping it is a chance to
typo it into creating a second account. root invoking the script directly offers
no default, since root is never the account being set up — and is refused if
typed.

The name is validated against the portable shape of a Linux account name before
anything else happens. Letting adduser refuse it later means several questions
have already been answered against a name that was never going to work.

It also no longer goes through prompt_value, which obeys any environment variable
matching the name it is filling in. USERNAME is set by some login environments,
and a variable this script silently takes as an answer should not be one that
might already be set for unrelated reasons. SETUP_USERNAME is the explicit
override.

The tmux config write moved out of the user section entirely. It was there only
because the original copied it right after adduser, where $USER_HOME first
exists. It is a dotfile and belongs with .zshrc and the starship config in the
shell section.

Verified: defaults to the sudo invoker, resolves daemon's home to /usr/sbin
rather than /home/daemon, falls back to /home for an account that does not exist,
and rejects a name with a space, a leading digit, one over 32 characters, and
root.

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

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

@@ -163,6 +163,62 @@ confirm() {
   done
 }
 
+# Which account this machine is being set up for.
+#
+# Asked at the top because two later questions default off it — where Officer is
+# installed, and where the disk ballast goes — so it has to be settled before
+# either is put to the user.
+#
+# Defaults to whoever invoked sudo. On a re-run, or on a machine that is already
+# somebody's, that is the answer every time, and typing it again is a chance to
+# typo it into creating a second account.
+#
+# SETUP_USERNAME in the environment answers it ahead of time. Deliberately not
+# USERNAME: that name is set by some login environments, and a variable this
+# script silently obeys should not be one that might already be in the
+# environment for unrelated reasons.
+ask_username() {
+  local default="${SUDO_USER:-}" answer
+
+  # root invoked the script directly rather than through sudo. It is never the
+  # account being set up, so there is nothing to suggest.
+  [[ "$default" == "root" ]] && default=""
+
+  if [[ -n "${SETUP_USERNAME:-}" ]]; then
+    answer="$SETUP_USERNAME"
+  else
+    echo ""
+    info "Which account is this machine for?"
+    echo "     The account you log in and work as — not root. It will be created"
+    echo "     if it does not exist."
+    echo ""
+    while [[ -z "${answer:-}" ]]; do
+      if ! read -rp "  Username${default:+ [$default]}: " answer; then
+        echo ""
+        fail "No answer. Set SETUP_USERNAME=<name> to answer this ahead of time."
+      fi
+      answer="${answer:-$default}"
+      [[ -z "$answer" ]] && warn "There is no default here — type a username."
+    done
+  fi
+
+  # The portable shape of a Linux account name. Worth checking rather than
+  # letting adduser refuse it later, because by then several questions have been
+  # answered against a name that was never going to work.
+  [[ "$answer" =~ ^[a-z_][a-z0-9_-]*\$?$ && ${#answer} -le 32 ]] ||
+    fail "'${answer}' is not a usable Linux username — lower case, starting with a letter or underscore."
+  [[ "$answer" == "root" ]] && fail "root is not the account to set up here."
+
+  USERNAME="$answer"
+
+  # Looked up, not assumed. The original built "/home/$USERNAME", which is merely
+  # the usual answer — an account created with a different home, or one whose home
+  # was moved, would have every later step writing to a directory that is not
+  # theirs.
+  USER_HOME="$(getent passwd "$USERNAME" 2>/dev/null | cut -d: -f6)"
+  [[ -n "$USER_HOME" ]] || USER_HOME="/home/${USERNAME}"
+}
+
 # Where Officer will live.
 #
 # Asked in pre-flight with the rest of the questions rather than at the point it