Add 'vaultik init' command and quickstart section in README

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.

internal/cli/entry_test.go

@@ -18,7 +18,7 @@ func TestCLIEntry(t *testing.T) {
 	}
 
 	// Verify all subcommands are registered
-	expectedCommands := []string{"snapshot", "store", "restore", "prune", "info", "version", "remote", "database"}
+	expectedCommands := []string{"init", "snapshot", "store", "restore", "prune", "info", "version", "remote", "database"}
 	for _, expected := range expectedCommands {
 		found := false
 		for _, cmd := range cmd.Commands() {