New init command writes a default config file with commented
explanations for every setting. Uses XDG config directory via
github.com/adrg/xdg for platform-appropriate paths:
macOS: ~/Library/Application Support/vaultik/config.yml
Linux: ~/.config/vaultik/config.yml
root: /etc/vaultik/config.yml
Config resolution now searches the XDG path before /etc/vaultik/.
Refuses to overwrite an existing file. Created with 0600 permissions.
README quickstart rewritten as a single copy-pasteable shell block
walking through install, keygen, init, edit, first backup, verify,
and cron setup.