docs: clarify agent skill writing guidance

[kristinapathak]

Goal

Document how agents should write public skills in NVIDIA/nvcf.

Scope

• Keep the public documentation guidance in one documentation-style skill.
• Make the skill apply to AGENTS.md, SKILL.md, GitHub issues, and GitHub Pull Request descriptions.
• Keep wording short, public-safe, and easy to scan.

Done when

• Root AGENTS points to the cross-tool skill fanout.
• The public skills catalog makes the skill easy to find.
• Skill guidance avoids internal-only process and temporary Pull Request status.