M3.3: Auto-import APT repo public.key during Calamares install

[mshafiee]

Acceptance criteria

• Calamares or post-install hook installs https://repo.zurvanlinux.org/public.key
• Key is stored in /etc/apt/keyrings/zurvan-archive-keyring.gpg
• No unsigned repository warnings on first apt update
• Pre-seed repo.zurvanlinux.org as a configured APT source in /etc/apt/sources.list.d/ during install so the custom repo is available out-of-the-box

Scope

Per project-next-steps.md M3.3 "Installer GPG + Repo Registration":

• Automatic public.key import during install.
• Pre-seeded repo.zurvanlinux.org source list.

Dependencies

• M1.2 publish pipeline stable
• public.key committed in apt-repository