Wire archinstall adapter + concrete phase implementations

Chunks 2-6 of the C3 refactor. With the adapter in place, the phase
list collapses from 12 stubs to 5 real phases:

  prepare_live          parse user_*.json into archinstall handlers
  arch_install          partition + base + bootloader + limine config +
                        early omarchy + useradd + runtime omarchy (all
                        inside a single Installer context manager)
  run_chroot_finalizer  arch-chroot -u $user → finalize.sh
  validate_boot         halt if /boot/limine.conf, /etc/kernel/cmdline,
                        or UKI are missing/malformed
  finish                reboot prompt

Key reordering vs upstream guided.py: write_limine_config runs BETWEEN
add_bootloader and the first add_additional_packages call. By the time
omarchy-limine pulls limine-mkinitcpio-hook (in the runtime package
install), /etc/default/limine and /etc/kernel/cmdline are in place, so
the UKI is built correctly on the first try — no stale-UKI purge,
no follow-up limine-update.

add_additional_packages is split into two calls so /etc/skel is
populated (via omarchy-settings in EARLY_PACKAGES) BEFORE useradd:

  EARLY_PACKAGES = base-devel git omarchy-keyring omarchy-settings omarchy-installer
  → installer.create_users(users)
  → runtime: omarchy + omarchy-base.packages

archinstall_adapter.py is the ONLY module that imports from archinstall.
If archinstall's API churns, the blast radius is contained here. Tested
against archinstall 4.3 / Python 3.14.

Limine template lookup tries the new install/assets/limine/ path first
and falls back to the legacy default/limine/ location during the
template-ownership migration (lands in a follow-up chunk).

install/orchestrator/main.py

@@ -33,32 +33,18 @@ def build_phases():
     """
     from .phases_impl import (
         prepare_live,
-        cleanup_disk,
-        partition_and_mount,
-        install_base,
-        install_bootloader,
-        write_limine_config,
-        install_early_omarchy_packages,
-        create_user,
-        install_omarchy_runtime,
+        arch_install,
         run_chroot_finalizer,
         validate_boot,
         finish,
     )
 
     return [
-        ("Preparing live environment",   prepare_live),
-        ("Cleaning install disk",        cleanup_disk),
-        ("Partitioning + mounting",      partition_and_mount),
-        ("Installing base system",       install_base),
-        ("Installing bootloader",        install_bootloader),
-        ("Writing Limine config",        write_limine_config),
-        ("Installing Omarchy keyring + settings", install_early_omarchy_packages),
-        ("Creating user",                create_user),
-        ("Installing Omarchy runtime",   install_omarchy_runtime),
-        ("Finalizing in chroot",         run_chroot_finalizer),
-        ("Validating boot setup",        validate_boot),
-        ("Finishing",                    finish),
+        ("Preparing live environment", prepare_live),
+        ("Installing Arch + Omarchy",  arch_install),
+        ("Finalizing in chroot",       run_chroot_finalizer),
+        ("Validating boot setup",      validate_boot),
+        ("Finishing",                  finish),
     ]