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/context.py

@@ -1,13 +1,16 @@
-"""Install context: parsed configurator output + invocation paths."""
+"""Install context: parsed configurator output, invocation paths, and a
+mutable `state` dict for objects that live across phases (e.g., the
+archinstall config handler and mirror list handler)."""
 
 from __future__ import annotations
 
 import json
-from dataclasses import dataclass
+from dataclasses import dataclass, field
 from pathlib import Path
+from typing import Any
 
 
-@dataclass(frozen=True)
+@dataclass
 class InstallContext:
     config_path: Path
     creds_path: Path
@@ -24,6 +27,10 @@ class InstallContext:
     log_path: Path = Path("/var/log/omarchy-install.log")
     target_log_path: Path = Path("/mnt/var/log/omarchy-install.log")
 
+    # Mutable per-run state shared across phases (e.g., 'arch_config_handler',
+    # 'mirror_handler'). Phases populate as needed; later phases read.
+    state: dict[str, Any] = field(default_factory=dict)
+
     @classmethod
     def from_args(cls, args) -> "InstallContext":
         config_path = Path(args.config)