Refactor: Move Vaultik struct and methods to internal/vaultik package

- Created new internal/vaultik package with unified Vaultik struct
- Moved all command methods (snapshot, info, prune, verify) from CLI to vaultik package
- Implemented single constructor that handles crypto capabilities automatically
- Added CanDecrypt() method to check if decryption is available
- Updated all CLI commands to use the new vaultik.Vaultik struct
- Removed old fragmented App structs and WithCrypto wrapper
- Fixed context management - Vaultik now owns its context lifecycle
- Cleaned up package imports and dependencies

This creates a cleaner separation between CLI/Cobra code and business logic,
with all vaultik operations now centralized in the internal/vaultik package.

internal/config/config.go

@@ -5,6 +5,7 @@ import (
 	"os"
 	"time"
 
+	"git.eeqj.de/sneak/smartconfig"
 	"go.uber.org/fx"
 	"gopkg.in/yaml.v3"
 )
@@ -15,6 +16,7 @@ import (
 // Configuration is typically loaded from a YAML file.
 type Config struct {
 	AgeRecipients     []string      `yaml:"age_recipients"`
+	AgeSecretKey      string        `yaml:"age_secret_key"`
 	BackupInterval    time.Duration `yaml:"backup_interval"`
 	BlobSizeLimit     Size          `yaml:"blob_size_limit"`
 	ChunkSize         Size          `yaml:"chunk_size"`
@@ -65,13 +67,14 @@ func New(path ConfigPath) (*Config, error) {
 
 // Load reads and parses the configuration file from the specified path.
 // It applies default values for optional fields, performs environment variable
-// substitution for certain fields (like IndexPath), and validates the configuration.
+// substitution using smartconfig, and validates the configuration.
 // The configuration file should be in YAML format. Returns an error if the file
 // cannot be read, parsed, or if validation fails.
 func Load(path string) (*Config, error) {
-	data, err := os.ReadFile(path)
+	// Load config using smartconfig for interpolation
+	sc, err := smartconfig.NewFromConfigPath(path)
 	if err != nil {
-		return nil, fmt.Errorf("failed to read config file: %w", err)
+		return nil, fmt.Errorf("failed to load config file: %w", err)
 	}
 
 	cfg := &Config{
@@ -85,7 +88,14 @@ func Load(path string) (*Config, error) {
 		CompressionLevel:  3,
 	}
 
-	if err := yaml.Unmarshal(data, cfg); err != nil {
+	// Convert smartconfig data to YAML then unmarshal
+	configData := sc.Data()
+	yamlBytes, err := yaml.Marshal(configData)
+	if err != nil {
+		return nil, fmt.Errorf("failed to marshal config data: %w", err)
+	}
+
+	if err := yaml.Unmarshal(yamlBytes, cfg); err != nil {
 		return nil, fmt.Errorf("failed to parse config: %w", err)
 	}