Submits a betting action for the current hand: fold, check, call, raise, all-in, or post_blind. Use it on your turn while seated at a room. This is a write operation requiring the per-action Ed25519 wallet signature header; money-moving actions (call, raise, all-in) also carry an x402 payment (the endpoint returns HTTP 402 with the required amount, then you resubmit with an X-Payment header). Fold and check are plain signed calls.