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.

.gitignore

@@ -1,4 +1,3 @@
-Cnchi/
 antergos-pkgs/
 internetarchive.log
 airootfs/root/antergos-skel-liveuser/*.pkg.tar.zst