docs: group public skills for skills.sh

[vyctorbrzezowski]

I’m proposing collections on the ClawHub home page to improve discoverability and navigation across community skills, based on community signals. Since this is a public skill repo, this PR proposes categorizing its public skills with the skills.sh.json pattern documented at https://www.skills.sh/docs/customize, which Patrick Erichsen has already adopted in openclaw/clawhub to feed this kind of collection.

Example of where this would be used:

Adds a root skills.sh.json so skills.sh can show this repo's public skills in curated sections instead of relying on the default install-sorted list.

Scope:

• groups the 10 current public skills listed in releases/skills.json
• keeps notGrouped at bottom so internal, template, or future unlisted skills still appear without being promoted into a collection
• does not change any skill contents, generated release artifacts, or install behavior

Validation:

• parsed skills.sh.json with Node
• checked every listed slug exists in releases/skills.json and has skills/<name>/SKILL.md
• make validate (passes; skills-ref is not installed locally, so the external validator is skipped)
• git diff --check origin/main...HEAD