Update SKILL.md to current skill metadata format

[elvaoao]

Summary

• add YAML frontmatter to SKILL.md with the skill name and description
• remove an outdated config.example.json reference
• clarify the web asset serving note

Why

Issue #50 reports that clawhub install clawfeed returns Skill not found.

This change updates the repository's SKILL.md from the older plain-markdown layout to the current metadata-based format used by OpenClaw/ClawHub skills, which appears to be a prerequisite for proper skill discovery and indexing.

I am not claiming this PR alone fully restores registry availability, since the current ClawHub registry still does not show a published clawfeed skill slug. But this change aligns the repo with the current skill file format and removes stale documentation from the skill file.

Validation

• invoked the clawfeed skill locally after this change and confirmed it was recognized and executed
• confirmed the repository SKILL.md now includes the same frontmatter pattern used by current working skills
• checked ClawHub registry on 2026-03-23: clawhub inspect clawfeed still returns Skill not found, so any full fix likely also requires republishing or registry sync

Related to #50