Add per-project prompt configuration UI in Codex Desktop

What problem would this solve?

In Codex Desktop, different projects often need different project-level instructions, conventions, or prompts. Today, users may need to manage these conventions manually through files such as AGENTS.md, which is less discoverable from the desktop UI and harder to configure consistently across projects.

Proposed solution

Add a project-level prompt configuration UI in Codex Desktop.

One possible interaction model:

1. Right-click a target project in the Codex Desktop project list.
2. Open a context menu item such as Settings or Configure.
3. Configure project-level instructions, prompts, or conventions for that specific project.
4. Persist the configuration as the project's AGENTS.md instructions, or otherwise keep it synchronized with the project-level instruction mechanism.

Why this would be useful

This would make project-specific guidance more discoverable and easier to maintain, especially for users who work across multiple repositories with different conventions, languages, review expectations, or workflow preferences.

Additional context

The configuration could map naturally to the existing AGENTS.md concept, while giving desktop users a first-class UI for creating and editing those project-level instructions.