feat: add Vercel integration

[pranavrajs]

Adds the Vercel integration (Engineering & status). Check Vercel projects, recent deployments, and whether the latest production deployment succeeded.

Tools

• List Projects (list_projects, GET)
• Get Project (get_project, GET)
• Get Latest Production Deployment (get_latest_production_deployment, GET)
• List Project Deployments (list_project_deployments, GET)
• List Recent Deployments (list_recent_deployments, GET)
• Get Deployment (get_deployment, GET)
• List Project Domains (list_project_domains, GET)

Testing

• The manifest passes the validator ("valid": true).
• Every template was rendered locally with Liquid 5.4.0 against sample responses built from Vercel's API docs.
• Not run against the live API. No credentials were available, so tool-tester.rb has not been run with a real key. This needs doing before merge.
• No logo is included.

Review notes

• No team ID input: the token should be scoped to a team or project. A full-account token only reaches personal-account projects.
• Tokens cannot be made read-only.

The root README.md row and .env.example entry for this integration are in a separate PR, so the per-integration PRs do not conflict with each other.

[pranavrajs]

Closing: developer tools are out of scope for this catalog.