refactor: split packages/ui/scripts/build-manifest.ts (1719 lines) into focused modules #625

packages/ui/scripts/build-manifest.ts is 1719 lines and mixes component extraction, non-component exports, tokens, surfaces, guides, demos, styling, markdown writers and the CLI.

Split into focused modules under packages/ui/scripts/manifest/ (components, exports, tokens, guides, demos, styling, migrations/writers) and keep build-manifest.ts as a thin CLI entry that re-exports the public API its importers use.

Proof: generated outputs are byte-identical before and after (dist/*.json, .agents/skills/codeatlas-ui/API.md, DEMOS.md, MIGRATION.md, site/public/api/v1/**, site/public/gallery/**), and the manifest tests stay green.