refactor: fold ghost-mcp into ghost-ui

[nahiyankhan]

Summary

• Fold the ghost-mcp package into packages/ghost-ui/src/mcp/. Its only job was re-exposing ghost-ui's registry.json over the MCP transport — no identity independent of ghost-ui, no separate consumers, no reason to carry a whole package.
• ghost-ui now ships the ghost-mcp bin directly. A new tsconfig.mcp.json builds the Node-only code to dist-mcp/ via tsc, kept out of the Vite lib pipeline. Drops the scripts/copy-assets.mjs step — data.ts reads registry.json and .shadcn/skills.md straight off disk via import.meta.url. Aligns on zod v4 (drops the v3 pin).
• Add per-package READMEs (ghost-ui, apps/docs) and a "Repo layout" section in the root README.md so a new reader can see the shape at a glance: main tool (ghost-drift) + reference design system (ghost-ui, which now ships the MCP bin) + docs site (apps/docs).
• Bin-only export (no ghost-ui/mcp subpath). Nothing imported from ghost-mcp as a library; the library surface can be added later in a 5-line PR if a real consumer appears.

Test plan

• pnpm install clean (MCP SDK added to ghost-ui; 115 transitive deps dropped along with ghost-mcp)
• pnpm --filter ghost-ui build:mcp succeeds; dist-mcp/ populated with bin + declarations
• Root pnpm build (composite tsc --build — now referencing packages/ghost-ui/tsconfig.mcp.json in place of the deleted packages/ghost-mcp) succeeds
• pnpm check (biome + typecheck + file-sizes + docs + cli-manifest) passes
• dist-mcp/bin.js responds to a stdio initialize handshake with the expected { tools, resources } capabilities
• Pre-commit and pre-push hooks (just check, just test, just build) all pass

No changeset — ghost-ui is in the changesets ignore list (private).

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