Add Codex SDK executor option

[guzus]

Motivation

• Provide an alternative executor that uses OpenAI Codex/Responses so the bot can run Codex-based workflows with tool calls.
• Keep the existing Anthropic SDK execution path while allowing runtime selection of a Codex executor via EXECUTOR_TYPE.
• Reuse the same task/event/action abstractions so UI/handlers and ralph-loop logic can remain unchanged.

Description

• Add a new CodexSdkExecutor implementation at src/services/CodexSdkExecutor.ts that implements a request/response loop against the OpenAI Responses API and executes local workspace tools (read_file, write_file, edit_file, list_directory, glob, grep, bash).
• Wire the codex option into the factory by extending ExecutorType in src/config/index.ts and returning CodexSdkExecutor from src/services/ExecutorFactory.ts when EXECUTOR_TYPE=codex.
• Document the new mode and authentication variables (CODEX_API_KEY / OPENAI_API_KEY) in README.md and add the Codex mode entry to the Executor Modes table.
• Implement a lightweight HTTP client inside the executor to call https://api.openai.com/v1/responses (uses CODEX_API_KEY/OPENAI_API_KEY) and translate responses into the project StreamEvent/action model.

Testing

• Ran bun run lint which completed successfully (ESLint warnings about TypeScript version but no errors).
• Ran bun run clean which completed successfully.
• Built the project with bun build src/index.ts --outdir dist --target node, which bundled successfully and produced the distribution.

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Codex Task