Refactor CLI to use flags instead of positional arguments

- Change all commands to use flags (--bucket, --prefix, etc.)
- Add --config flag to backup command
- Support VAULTIK_CONFIG environment variable for config path
- Use /etc/vaultik/config.yml as default config location
- Add test/config.yaml for testing
- Update tests to use environment variable for config path
- Add .gitignore for build artifacts and local configs
- Update documentation to reflect new CLI syntax

internal/cli/prune.go

@@ -22,17 +22,24 @@ func NewPruneCommand() *cobra.Command {
 	opts := &PruneOptions{}
 
 	cmd := &cobra.Command{
-		Use:   "prune <bucket> <prefix>",
+		Use:   "prune",
 		Short: "Remove unreferenced blobs",
 		Long:  `Delete blobs that are no longer referenced by any snapshot`,
-		Args:  cobra.ExactArgs(2),
+		Args:  cobra.NoArgs,
 		RunE: func(cmd *cobra.Command, args []string) error {
-			opts.Bucket = args[0]
-			opts.Prefix = args[1]
+			// Validate required flags
+			if opts.Bucket == "" {
+				return fmt.Errorf("--bucket is required")
+			}
+			if opts.Prefix == "" {
+				return fmt.Errorf("--prefix is required")
+			}
 			return runPrune(cmd.Context(), opts)
 		},
 	}
 
+	cmd.Flags().StringVar(&opts.Bucket, "bucket", "", "S3 bucket name")
+	cmd.Flags().StringVar(&opts.Prefix, "prefix", "", "S3 prefix")
 	cmd.Flags().BoolVar(&opts.DryRun, "dry-run", false, "Show what would be deleted without actually deleting")
 
 	return cmd