docs(readme): Interchange-style install and examples (CL-8775)

[TheGreatAxios]

Summary

The Quickstart now leads with how this package is actually consumed in production: registering its exports as one entry in a host's OAuthLoginProviders map, passed to @corbits/oauth-core/hub's mountOAuthLogin and createOAuthTokenRefresher. That's the real pattern the Interchange hub (workbench) uses for "Continue with Codex" — this package never runs its own callback server, token store, or refresh loop; oauth-core owns all of that.

The OAuth entry shows the real signatures:

• refresh only ever receives the stored refresh secret, so refreshCodexTokens's third argument (the "previous" tokens) is seeded with just that secret
• metadata extracts accountId from the exchanged tokens, since the Codex backend rejects inference without that header

Inference-adapter registration (createCodexResponsesAdapter, withCodexContentTypeRepair) moves to its own "Register the inference adapter" subsection, unchanged from before.

What changed

• No in-memory Maps, fake token stores, or standalone re-wiring of this package's own exports into itself
• Every export name in the README exists in src/ (CODEX_PROVIDER, codexOAuthConfig, exchangeCodexCode, refreshCodexTokens, createCodexResponsesAdapter, withCodexContentTypeRepair)
• Both code fences were verified against tsc --noEmit in this repo before committing