Add codex agent skills setup

[yoRyuuuuu]

Custom prompts in Codex are now deprecated. Codex recommends using Agent Skills instead.
This PR adds the setup commands for Codex Agent Skills, so new setups follow the recommended approach.

https://developers.openai.com/codex/custom-prompts/#:~:text=Deprecated.%20Use%20skills%20for%20reusable%20prompts

[gotalab]

@yoRyuuuuu

Thanks for the great PR!

Could you please apply a few changes?

1. Move the skills directory from .codex/skills to .agents/skills.
2. Remove the frontmatter metadata (short description) from SKILL.md.
3. Add agents/openai.yml in each skill directory as an alternative metadata file.

Reference:
https://developers.openai.com/codex/skills
https://developers.openai.com/codex/skills/create-skill#advanced-configuration

If you'd like, I can also push these changes to your branch.

[gotalab]

@yoRyuuuuu

I can’t merge it because it modifies cc-sdd’s core instructions and they now differ from the intended custom prompts contract. That would change behavior for users.

I'm going to close this PR. If you'd like, please open a new PR that keeps the instcutions unchanged. For instruction changes, let's discuss via an issue first.