R.I.P. Cnchi (2012-2026) — Calamares is the big boy installer now

Full removal of Cnchi from the ISO. It was the original Antergos
installer and we tried to keep it alive with a GTK4 port, but the
Python GTK4 bindings were unstable, the maintenance burden was
unsustainable for a solo dev, and Calamares simply does everything
better.

What changed:
  - README: complete rewrite of installer section with eulogy and
    'Why Calamares and not Cnchi' explanation
  - packages.x86_64: cnchi removed (calamares stays)
  - run_before_squashfs.sh: removed cnchi memes dir creation, cnchi
    desktop file fix, cnchi version comment; changed GTK theme from
    Adwaita to Adwaita-dark (fixes white-on-white welcome app)
  - airootfs/.../cnchi.desktop: deleted
  - user-commands-before.bash: updated cnchi references → generic
  - user_commands.bash: updated cnchi reference → generic
  - .gitignore: removed Cnchi/ entry
  - AGENTS.md: cnchi → calamares
  - .github/workflows/build.yml: cnchi → calamares in README gen
  - NOTICES: kept historical Cnchi credit (unchanged)

🕯️ Requiescat in pace, Cnchi. You did your job.

airootfs/root/antergos-skel-liveuser/src/etc/skel/.local/share/applications/cnchi.desktop

@@ -1,14 +0,0 @@
-[Desktop Entry]
-Type=Application
-Version=1.0
-Name=Install Antergos
-GenericName=System Installer
-Comment=Cnchi — Antergos System Installer
-Keywords=cnchi;system;installer;
-TryExec=cnchi
-Exec=sudo -E cnchi
-Categories=Qt;System;
-Icon=cnchi
-Terminal=false
-SingleMainWindow=true
-StartupNotify=true

airootfs/root/antergos-skel-liveuser/src/etc/skel/user-commands-before.bash

@@ -1,11 +1,11 @@
 #!/bin/bash
 #
 # This file can contain commands that will be executed (as root) before
-# cnchi is started. So here you may configure various settings of cnchi.
+# Calamares is started. So here you may configure various settings.
 #
 # NOTE! This is an advanced feature, so you need to know *exactly* what
 # you are doing. Otherwise bad things can happen...
-# Sound knowledge of cnchi, bash language, and linux commands is required
+# Sound knowledge of bash language and linux commands is required
 # for a meaningful customization.
 #
 # If you add commands to this file, start the whole install process fresh
@@ -24,7 +24,7 @@
 
 _IsoConfig() {
     ## Add your ISO configurations here.
-    ## This is executed as root before cnchi is started.
+    ## This is executed as root before the installer is started.
 
     local -r install_mode="$1"          # 'online' or 'offline'
 

airootfs/root/antergos-skel-liveuser/src/etc/skel/user_commands.bash

@@ -50,7 +50,7 @@
 
 _PostInstallCommands() {
     ## Add your "late-install" commands here.
-    ## This is executed as root near the end of cnchi execution.
+    ## This is executed as root near the end of installation.
 
     local -r username="$1"              # new user you created for the target