Add an explicit repository license

[UTXOnly]

Problem

The repository has no LICENSE file. Public source without an explicit license does not clearly grant users permission to use, modify, or redistribute the client, which limits adoption despite the package being installable from GitHub.

Acceptance criteria

• Select and add an explicit license approved by the repository owner.
• Add a short license reference to the README.
• Clarify whether the vendored Kalshi OpenAPI and AsyncAPI specifications are covered by separate upstream terms if necessary.
• Package and release metadata remain consistent with the selected license.