Release v2.7.2

improve OpenApiClient types, add E2E tests, CI, and README

- Rewrite OpenApiClient as a structural interface with typed request
  options, all client members (getConfig, buildUrl, response interceptor,
  eject), and method syntax for bivariant compat with bundled codegen client
- Remove @hey-api/client-fetch dep (bundled since openapi-ts v0.73.0)
- Add E2E test: generates SDK from petstore spec, verifies type compat
  with OpenApiClient, configures client, makes HTTP requests
- Add GitHub Actions CI (test + version tagging) and release workflow
- Add README documenting motivation, generation, client config, file
  uploads, error handling, and response utilities
- Extract build script to scripts/build.sh, fix cross-platform sed→perl
- Replace ts-node with tsx

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>