npm SDK: implement core payment flows

## Summary

  Implement the core OmniClaw flows in the npm SDK.

  ## Scope

  Initial parity target:
  - wallet creation / listing
  - simulate
  - pay
  - payment intents
  - trust lookup

  ## Acceptance criteria

  - typed methods exist for all core flows
  - request and response models are documented
  - error handling is typed and consistent
  - at least one working example is included
