Creates entity model documents with Mermaid.js ER diagrams and attribute tables defining entities, relationships, data types, and validation rules. Use when the user asks to "create an entity model", "design a data model", "draw an ERD", "define database schema", "model entities", or mentions entity-relationship diagram, ER diagram, database design, or data modeling. Also trigger whenever the task is to produce, write, or design an entity model or data model document (e.g. docs/entity_model.md) from a requirements catalog — including phrasings like "data model design", "the team needs an entity model before development", or a request to document entities with attributes, data types, precision, and validation or cross-attribute constraints.