fix: add missing cursor field to snapshot command schema (#435)

The `-C`/`--cursor` flag was added to the CLI parser and snapshot
implementation in #374, but the Zod schema in protocol.ts was not
updated. This caused the `cursor` field to be silently stripped
during command validation, so cursor-interactive element detection
never ran.

Fixes #434

src/protocol.ts

@@ -719,6 +719,7 @@ const screenshotSchema = baseCommandSchema.extend({
 const snapshotSchema = baseCommandSchema.extend({
   action: z.literal('snapshot'),
   interactive: z.boolean().optional(),
+  cursor: z.boolean().optional(),
   maxDepth: z.number().nonnegative().optional(),
   compact: z.boolean().optional(),
   selector: z.string().optional(),