feat: support passing Github issue as a context

What is this issue for

To track the work for supporting Github issue as an input or context for Codex

Why

While Codex is built to help devs with their code locally, in most real-world workflows, devs start by addressing a GitHub issue — just like this one. Enabling Codex to understand the context of an issue, clone the relevant repository, and work toward resolving the problem or GH issue would significantly boost its usefulness and alignment with real dev usecases. For example, SWE Agent, which is currently SOTA on SWE-bench full supports this functionality.