feat: implement --prune flag on snapshot create (closes #4)

The --prune flag on 'snapshot create' was accepted but silently did nothing
(TODO stub). This connects it to actually:

1. Purge old snapshots (keeping only the latest) via PurgeSnapshots
2. Remove unreferenced blobs from storage via PruneBlobs

The pruning runs after all snapshots complete successfully, not per-snapshot.
Both operations use --force mode (no interactive confirmation) since --prune
is an explicit opt-in flag.

Moved the prune logic from createNamedSnapshot (per-snapshot) to
CreateSnapshot (after all snapshots), which is the correct location.

internal/vaultik/snapshot.go

@@ -558,7 +558,7 @@ func (v *Vaultik) PurgeSnapshots(keepLatest bool, olderThan string, force bool)
 	if !force {
 		v.printfStdout("\nDelete %d snapshot(s)? [y/N] ", len(toDelete))
 		var confirm string
-		if _, err := fmt.Scanln(&confirm); err != nil {
+		if _, err := v.scanStdin(&confirm); err != nil {
 			// Treat EOF or error as "no"
 			v.printlnStdout("Cancelled")
 			return nil

internal/vaultik/vaultik.go

@@ -129,7 +129,7 @@ func (v *Vaultik) GetFilesystem() afero.Fs {
 	return v.Fs
 }
 
-// printfStdout writes formatted output to stdout for user-facing messages.
+// printfStdout writes formatted output to stdout.
 func (v *Vaultik) printfStdout(format string, args ...any) {
 	_, _ = fmt.Fprintf(v.Stdout, format, args...)
 }