Implement global skill enablement

[Che-Zhu]

Summary

• add global UserSkill state and INSTALL_SKILL project tasks
• add the /skills UI and enable flow, and inherit enabled skills on new projects
• add skill install execution inside sandboxes and fix reconcile/startup stability issues found during verification

Scope

• includes global skill enablement only
• does not implement uninstall/disable yet

Verification

• pnpm lint
• pnpm exec tsc --noEmit
• manually verified enabling frontend-design from /skills and observed the install task succeed in the sandbox

[github-actions]

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  Commit: 3fe9a47453810470edbdae9d7c51bcd07f4e8b5a
  Branch: codex/global-skill-enablement

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