docs: update skill install instructions to use npx skills add

Created by: gatteo

Summary

• Replace ClawHub install commands with npx skills add tetherto/wdk-agent-skills in both ai/openclaw.md and ai/agent-skills.md
• Update comparison table, platform table, and resources links in agent-skills.md
• Add info hint that the skill will also be published on ClawHub shortly

Test plan

• Verify the install command npx skills add tetherto/wdk-agent-skills renders correctly
• Verify all updated links and references are accurate
• Confirm ClawHub hint displays properly in GitBook