Initial chezmoi-managed dotfiles with bootstrap scripts

- Rename master to legacy-2025 on remote (frozen pre-chezmoi snapshot)
- New orphan 'main' branch with bootstrap-enabled config
- .chezmoi.yaml.tmpl detects os_family (debian | arch) from /etc/os-release
- dot_zshrc.tmpl refactored from current miche config with os_family
  conditional for pacman vs apt aliases
- dot_config/: bat, btop, ghostty (with gruvbox themes), kitty (with gruvbox
  colors), nvim (LazyVim), paru
- dot_gitconfig.tmpl, dot_tmux.conf (verbatim from current state)
- run_once_00-install-bootstrap-tools.sh.tmpl: age, git, curl, ca-certificates
- run_once_10-add-chaotic-aur.sh.tmpl (arch-only): add Chaotic-AUR + install paru
- run_once_20-install-user-packages.sh.tmpl: zsh, tmux, neovim (with version
  check + binary tarball fallback for debian), oh-my-zsh + plugins, tpm,
  rustup, all CLI tools
- run_onchange_30-ensure-cargo.sh.tmpl: rustup fallback
- README.md with onboarding runbook

dot_config/nvim/lua/plugins/multicursor.lua

@@ -0,0 +1,4 @@
+return {
+  "mg979/vim-visual-multi",
+  event = "VeryLazy", -- Load on startup
+}