chore(agents): Add skill-scanner skill

[github-actions]

Note

The pull request "chore(agents): Add skill-scanner skill" was created by @chargome but did not reference an issue. Therefore this issue was created for better visibility in external tools like Linear.

Uses dotagents to add the skill-scanner skill from getsentry/skills for scanning agent skills for security issues such as prompt injection, malicious scripts and supply chain risks.

[linear]

JS-1848