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.

.github/workflows/build.yml

@@ -130,14 +130,14 @@ jobs:
         echo "## Installation" >> README.md
         echo "1. Boot from ISO" >> README.md
         echo "2. Select desktop environment" >> README.md
-        echo "3. Run Cnchi installer" >> README.md
+        echo "3. Run Calamares installer" >> README.md
         echo "" >> README.md
         echo "## Issues" >> README.md
         echo "Report at: https://github.com/Antergos-NeXT/antergos-iso/issues" >> README.md
         echo "" >> README.md
         echo "## Source" >> README.md
         echo "- ISO Build: https://github.com/Antergos-NeXT/antergos-iso" >> README.md
-        echo "- Cnchi: https://github.com/Antergos-NeXT/cnchi-next" >> README.md
+        echo "- Calamares: https://codeberg.org/calamares/calamares" >> README.md
         echo "- Packages: https://github.com/Antergos-NeXT/antergos-packages" >> README.md
 
         # Upload ISO + README