refactor: formalize graph-plugins-shared public API surface

[Yonava]

Summary

• Restructures graph-plugins-shared so every subdirectory (plugins/, theme/, computed-tokens/, drag/) has an index.ts as the sole public entry point, with all implementation files moved into internals/
• Adds explicit package exports entries for each subdirectory so imports go through the index
• Renames internal types for clarity: LoosePluginData → LoosePluginSchema, GraphPluginOptions → PluginInput, ResolvedGraphPlugin return shape → PluginOutput, ThemesForPlugins → PluginThemes
• JSDoc on all public exports, segmented by audience (plugin authors vs. graph orchestrator)
• Sweeps all consumers (graph-plugins, graph-theme-presets, create-graph, products) off deep/dist imports and onto the new public indexes