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/root.go

@@ -3,7 +3,9 @@ package cli
 import (
 	"fmt"
 	"os"
+	"path/filepath"
 
+	"github.com/adrg/xdg"
 	"github.com/spf13/cobra"
 )
 
@@ -32,13 +34,14 @@ on the source system.`,
 	}
 
 	// Add global flags
-	cmd.PersistentFlags().StringVar(&rootFlags.ConfigPath, "config", "", "Path to config file (default: $VAULTIK_CONFIG or /etc/vaultik/config.yml)")
+	cmd.PersistentFlags().StringVar(&rootFlags.ConfigPath, "config", "", "Path to config file (default: $VAULTIK_CONFIG or platform config dir)")
 	cmd.PersistentFlags().BoolVarP(&rootFlags.Verbose, "verbose", "v", false, "Enable verbose output")
 	cmd.PersistentFlags().BoolVar(&rootFlags.Debug, "debug", false, "Enable debug output")
 	cmd.PersistentFlags().BoolVarP(&rootFlags.Quiet, "quiet", "q", false, "Suppress non-error output")
 
 	// Add subcommands
 	cmd.AddCommand(
+		NewInitCommand(),
 		NewRestoreCommand(),
 		NewPruneCommand(),
 		NewStoreCommand(),
@@ -59,25 +62,41 @@ func GetRootFlags() RootFlags {
 }
 
 // ResolveConfigPath resolves the config file path from flags, environment, or default.
-// It checks in order: 1) --config flag, 2) VAULTIK_CONFIG environment variable,
-// 3) default location /etc/vaultik/config.yml. Returns an error if no valid
-// config file can be found through any of these methods.
+// Search order: --config flag, VAULTIK_CONFIG env, XDG config dir, /etc/vaultik/config.yml.
 func ResolveConfigPath() (string, error) {
-	// First check global flag
 	if rootFlags.ConfigPath != "" {
 		return rootFlags.ConfigPath, nil
 	}
 
-	// Then check environment variable
 	if envPath := os.Getenv("VAULTIK_CONFIG"); envPath != "" {
 		return envPath, nil
 	}
 
-	// Finally check default location
-	defaultPath := "/etc/vaultik/config.yml"
-	if _, err := os.Stat(defaultPath); err == nil {
-		return defaultPath, nil
+	for _, path := range defaultConfigPaths() {
+		if _, err := os.Stat(path); err == nil {
+			return path, nil
+		}
 	}
 
-	return "", fmt.Errorf("no config file found; specify one with --config, set VAULTIK_CONFIG, or create %s", defaultPath)
+	return "", fmt.Errorf("no config file found; run 'vaultik init' to create one, or specify with --config")
+}
+
+// defaultConfigPaths returns the ordered list of config paths to search.
+// On macOS: ~/Library/Application Support/vaultik/config.yml
+// On Linux: ~/.config/vaultik/config.yml
+// Fallback: /etc/vaultik/config.yml
+func defaultConfigPaths() []string {
+	return []string{
+		filepath.Join(xdg.ConfigHome, "vaultik", "config.yml"),
+		"/etc/vaultik/config.yml",
+	}
+}
+
+// DefaultConfigPath returns the platform-appropriate default config path.
+// Used by the init command and in help text.
+func DefaultConfigPath() string {
+	if os.Getuid() == 0 {
+		return "/etc/vaultik/config.yml"
+	}
+	return filepath.Join(xdg.ConfigHome, "vaultik", "config.yml")
 }