Refuse to run the Linux setup as root

setup-linux.sh accepted being run as root and simply set SUDO="". That quietly
broke everything user-scoped: PM2 keeps its process list in $HOME/.pm2 and the
login entry goes in $HOME/.config/autostart, so under sudo both landed in /root.
The install reported success, then `pm2 list` as the real user showed nothing -
it was talking to a different daemon - and the autostart entry sat in a home
directory that never logs in.

Both scripts now refuse, and name the account to use when SUDO_USER says how
they were invoked. setup-linux.sh still uses sudo for apt, which is the only
part that needs it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

scripts/install-autostart.sh

@@ -31,6 +31,16 @@ done
 info() { printf '  %s\n' "$1"; }
 warn() { printf '  ! %s\n' "$1" >&2; }
 
+# Same reason as setup-linux.sh: PM2's process list lives in $HOME/.pm2 and the
+# login entry in $HOME/.config/autostart. As root both go to /root, invisible to
+# the account that actually logs in.
+if [ "$(id -u)" -eq 0 ]; then
+    echo "  [X] do not run this as root${SUDO_USER:+ - run it as $SUDO_USER}." >&2
+    echo "      PM2 and the login entry are per-user; as root they land in /root" >&2
+    echo "      and your own 'pm2 list' will be empty." >&2
+    exit 1
+fi
+
 info "project root: $ROOT"
 
 # ── PM2 ─────────────────────────────────────────────────────────────────────