chore: streamline macmini config, add Ghostty, fix SSH/age setup
- Remove redundant media codec brews (covered by ffmpeg in nix packages)
- Remove gaming casks (epic-games, roblox, ea) and libreoffice
- Remove Steam Link (requires Rosetta 2) and OP Auto Clicker from MAS
- Add Ghostty as declarative terminal across all desktop hosts
- Replace Terminal.app with Ghostty in Dock
- Fix SSH keychain launchd agent flag (--apple-use-keychain)
- Update macmini SSH public key
- Update sops ewan age key
- Add verify-ssh-external script for GitHub, Forgejo, Tangled
- Fix check-secrets.sh to not require git and handle dotenv format
- Suppress home-manager manual warnings (HM issue #7935)
- Ensure nix-config is always a git repo with correct remotes on activation