Add Arch Linux CT support and pacman integration

Add Arch Linux container support and integrate pacman into install utilities. New files: ct/archlinux.sh (container build/update script), install/archlinux-install.sh (install entrypoint), and json/archlinux.json (metadata). misc/install.func: detect_arch in detect_os, add pacman bootstrap, and implement pacman handlers for pkg_upgrade, pkg_install, pkg_remove, pkg_clean and package refresh; include openssh install path for pacman and provide a cleanup_lxc override to run pkg_clean. Enables creating/updating Arch Linux LXC containers and proper package manager handling.

json/archlinux.json

@@ -0,0 +1,38 @@
+{
+  "name": "Arch Linux",
+  "slug": "archlinux",
+  "categories": [2],
+  "date_created": "2026-04-27",
+  "type": "ct",
+  "updateable": true,
+  "privileged": false,
+  "interface_port": null,
+  "documentation": "https://wiki.archlinux.org/",
+  "website": "https://archlinux.org/",
+  "logo": "https://cdn.jsdelivr.net/gh/selfhst/icons@main/webp/arch-linux.webp",
+  "description": "Arch Linux is a lightweight, rolling-release Linux distribution that follows a keep-it-simple philosophy. It provides a minimalist base system that users can build upon according to their needs.",
+  "install_methods": [
+    {
+      "type": "default",
+      "script": "ct/archlinux.sh",
+      "config_path": null,
+      "resources": {
+        "cpu": 1,
+        "ram": 512,
+        "hdd": 4,
+        "os": "Arch Linux",
+        "version": "base"
+      }
+    }
+  ],
+  "default_credentials": {
+    "username": null,
+    "password": null
+  },
+  "notes": [
+    {
+      "text": "Arch Linux is a rolling-release distribution. Run 'pacman -Syu' regularly to keep the system up to date.",
+      "type": "info"
+    }
+  ]
+}