chore(skills): track third-party skills in skills-lock.json (#15724)

* chore(skills): track third-party skills in skills-lock.json

Install gh-stack, mintlify, and vercel-react-best-practices through the
skills CLI so they join agent-browser in skills-lock.json, and refresh all
four from upstream. A lockfile entry is what hides a vendored skill from
`npx skills add Arize-ai/phoenix`, so the hand-added `internal: true`
flags are no longer needed and would be reverted by the weekly update.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* chore(skills): drop third-party section from skills README

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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Co-authored-by: Claude Fable 5 <noreply@anthropic.com>