Make setup ISO-only

Remove legacy online installer entrypoints, collapse migrations for 4.0, and move setup responsibilities into target-side system, hardware, and user commands.

install/hardware/pacman.sh

@@ -0,0 +1,12 @@
+# Hardware-specific pacman repository extensions that must survive the final
+# pacman.conf restore.
+if lspci -nn | grep -q "106b:180[12]"; then
+  if ! grep -q '^\[arch-mact2\]' /etc/pacman.conf; then
+    cat >> /etc/pacman.conf <<'EOF'
+
+[arch-mact2]
+Server = https://github.com/NoaHimesaka1873/arch-mact2-mirror/releases/download/release
+SigLevel = Never
+EOF
+  fi
+fi