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_prune_test.go

@@ -0,0 +1,23 @@
+package vaultik
+
+import (
+	"testing"
+)
+
+// TestSnapshotCreateOptions_PruneFlag verifies the Prune field exists on
+// SnapshotCreateOptions and can be set.
+func TestSnapshotCreateOptions_PruneFlag(t *testing.T) {
+	opts := &SnapshotCreateOptions{
+		Prune: true,
+	}
+	if !opts.Prune {
+		t.Error("Expected Prune to be true")
+	}
+
+	opts2 := &SnapshotCreateOptions{
+		Prune: false,
+	}
+	if opts2.Prune {
+		t.Error("Expected Prune to be false")
+	}
+}