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/size.go

@@ -51,3 +51,12 @@ func (s Size) Int64() int64 {
 func (s Size) String() string {
 	return humanize.Bytes(uint64(s))
 }
+
+// ParseSize parses a size string into a Size value
+func ParseSize(s string) (Size, error) {
+	bytes, err := humanize.ParseBytes(s)
+	if err != nil {
+		return 0, fmt.Errorf("invalid size format: %w", err)
+	}
+	return Size(bytes), nil
+}