## What changed
Add `codex-runtime-agent-instance` with serializable registration requests and deserializable responses. Requests support OAuth, personal access tokens, and platform API keys, along with an installation ID, public key, harness metadata, and runtime role. Responses include runtime and host IDs plus ChatGPT or platform actor context.
Use `RedactedString` for credentials to keep secrets out of debug output.
## Testing
Add tests covering request serialization for all three credential types, debug redaction, response decoding for both actor types, and rejection of responses missing required fields.
GitOrigin-RevId: f33bae734fdd1402144dee407e5b868f5eef1d37
