Three changes:
1. NEW run_once_05-install-pacstall.sh.tmpl (debian-only)
Installs pacstall via its official installer. Pacstall is an
AUR-like package manager for debian/ubuntu, with neovim at
0.12.2-1 (current as of bootstrap). The installer requires
root and prompts for 'install axel?', so we run it under
sudo with NON_INTERACTIVE=true and stdin redirected from /dev/null.
2. UPDATE run_once_20-install-user-packages.sh.tmpl
On debian, prefer pacstall over the GitHub tarball when
pacstall is available. The tarball fallback remains for the
case where pacstall install failed or isn't wanted.
3. NEW dot_config/topgrade/topgrade.toml
Topgrade's built-in pacstall step auto-detects pacstall and
runs 'pacstall -U -Up' (update repo + upgrade packages).
Built-in chezmoi step also auto-detects chezmoi. So our
topgrade config just sets pre_sudo=true for password caching
and ignore_failures for node.
