feat: export schema API for building custom tools and add richText lexical features to schema

[jhb-dev]

• feat: the plugin entry now exports the schema API for building custom tools (e.g. listEntities + getEntitySchema + getBlockSchema MCP tools) without going through HTTP. listReadableEntities({ req }) returns the readable collection/global slugs plus localization; getEntitySchema({ req, type, slug }) returns the same { slug, fields, jsonSchema } the /schema endpoint produces for one entity; getBlockSchema({ req, slugs }) resolves richText block slugs to { slug, fields }.
• feat: schema endpoint now reports the enabled Lexical nodes of each richText field under a lexicalFeatures key, organized by where each node lives in richtext JSON. Every key is the exact node type to emit, so agents know what a field accepts before authoring. Surfaces in _schema.json.
• breaking: renamed the exported extractFields to toFieldSchemas