feat: enable subfolder-based custom commands

[mickitty0511]

Feature Explanation

This program allows codex to recognize md files in sub-folders of ~/.codex/prompts as custom commands
For codex users discussing Issue #4735, who may know major CLIs can recognize md files in sub-folders as custom commands, I've made the same feature.

Now, we are organizing complex customs commands in the following structure.
This development-style is growing further than ever, and Codex should allow it.

prompts/                  
├── make-article.md          # custom command
└── article-strcutures/      # sub-folder
    ├── seo-friendly.txt         # SEO-specific format
    └── tech-post.txt            # tech-post format

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[mickitty0511]

I have read the CLA Document and I hereby sign the CLA