release(mcp): make the MCP server package installable and registry-ready

[7shep]

Problem

@universal/design-mcp builds a local binary, but the package is private and lacks release-ready metadata and an install smoke test. Users must clone the monorepo before connecting Universal to an MCP client.

Suggested level: Intermediate to advanced
Primary area: packages/design-mcp and release docs

Acceptance criteria

• Define the approved public package name, files, license, repository, keywords, engines, and executable metadata.
• Ensure pnpm pack contains only required runtime files and docs.
• Smoke-test installing the tarball in a temporary fixture and starting the MCP binary without monorepo source paths.
• Document package-manager/MCP-client configuration.
• Add a valid draft server.json or generation step for the official MCP Registry namespace.
• Keep publishing maintainer-triggered and credential-free in PRs.
• Document versioning, rollback, and pre-release expectations.
• MCP and packaging checks pass.

Out of scope

• Publishing under an unapproved name
• Committing npm or registry credentials
• Promising stable APIs before a stable release