[codex] align SDK API documentation workflow

[KazanderDad]

What changed

• Renamed the SDK surface to MyPayTag and aligned public protocol/docs with the current resolver direction.
• Added OpenAPI 3.1 as the public API source of truth at api/openapi.yaml.
• Added generated Postman collection support at api/postman_collection.json.
• Added CI/release handling for OpenAPI validation and Postman collection artifacts.
• Stabilized Postman generation so rerunning the generator does not create noisy random diffs.

Why

Developers need one maintainable API documentation source, interactive docs on the site, and a downloadable Postman collection without adding heavier doc platforms or package publishing changes in this stage.

Validation

• pnpm test
• pnpm api:validate
• pnpm api:postman
• double-generation diff check for api/postman_collection.json
• git diff --check

[KazanderDad]

@codex review