docs: rewrite README to the Corbits README standard

[TheGreatAxios]

Closes CL-9406

Summary

Rewrites the README to the Corbits README standard:

• Opener naming what the package is and where it plugs into @intx/inference, then "Why @corbits/codex-provider?" with three points.
• One bun add line with the peers, in place of the npm/pnpm/yarn/bun list.
• Quickstart: one real turn through runInference with createCodexResponsesAdapter and withCodexContentTypeRepair, printing the streamed text. Needs CODEX_ACCESS_TOKEN and CODEX_ACCOUNT_ID, and throws if either is unset. A comment explains why deps is built directly: createDependencies binds the global fetch, so it cannot install the content-type repair.
• Where it fits → Reference (every export) → Using with Interchange (the OAuthLoginProviders entry and an AdapterManifest entry with the wrapped fetch) → License.
• "Runtime support", "How it works" and "Development" are removed; the runtime line moves under Install, and development notes move to CONTRIBUTING later in this stack.

Checks

Every ts block typechecks against src/. bun run check passes.