feat: replace Debian Live with custom debootstrap ISO base + branding

Major ISO build overhaul on dev-iso branch:

- Replace ~800MB Debian Live download with debootstrap --variant=minbase
  (~150MB installer squashfs built from scratch)
- Custom initramfs with archipelago-mount hook for boot media detection
- Systemd service auto-starts installer (replaces profile.d hack)
- GRUB + ISOLINUX configs written from scratch (no Debian Live dependency)
- EFI boot image built with grub-mkimage (no more MBR extraction)
- Archipelago GRUB theme: dark background, Bitcoin orange accents
- Theme installed on both installer ISO and target system
- Rootfs optimizations: --no-install-recommends, strip docs/man/locales,
  remove firmware-misc-nonfree/wget/htop, add explicit font deps
- Separate CI workflow (build-iso-dev.yml) for dev-iso branch
- Includes pre-existing fixes from main (build-iso.yml, middleware, Login)

Target: sub-2GB unbundled ISO (down from 3.9GB)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

image-recipe/branding/grub-theme/theme.txt

@@ -0,0 +1,52 @@
+# Archipelago GRUB Theme
+# Dark background with Bitcoin orange accents
+
+title-text: ""
+desktop-color: "#0a0a0a"
+terminal-font: "DejaVu Sans Regular 16"
+
++ boot_menu {
+    left = 25%
+    top = 40%
+    width = 50%
+    height = 30%
+    item_font = "DejaVu Sans Regular 16"
+    item_color = "#aaaaaa"
+    selected_item_font = "DejaVu Sans Regular 16"
+    selected_item_color = "#ffffff"
+    selected_item_pixmap_style = "select_*.png"
+    item_height = 36
+    item_spacing = 8
+    item_padding = 16
+    scrollbar = false
+}
+
++ label {
+    left = 25%
+    top = 20%
+    width = 50%
+    text = "A R C H I P E L A G O"
+    font = "DejaVu Sans Bold 24"
+    color = "#f7931a"
+    align = "center"
+}
+
++ label {
+    left = 25%
+    top = 28%
+    width = 50%
+    text = "Bitcoin Node OS"
+    font = "DejaVu Sans Regular 14"
+    color = "#888888"
+    align = "center"
+}
+
++ label {
+    left = 25%
+    top = 90%
+    width = 50%
+    text = "Use arrow keys to select, Enter to boot"
+    font = "DejaVu Sans Regular 12"
+    color = "#555555"
+    align = "center"
+}