Declare an entity-and-relationship ontology for graphiti's custom entity extraction.

A consumer writes:

defmodule MyOntology do
  use Gralkor.Ontology, entities: :strict, relationships: :scoped

  entity Teammate, "A person the agent works with." do
    field :handle,   :string, required: true, doc: "stable login handle"
    field :timezone, :string,                  doc: "IANA tz"
  end

  entity WorkingPreference, "A way a teammate prefers to work." do
    field :description, :string, required: true
  end

  from Teammate do
    prefers WorkingPreference do
      field :since, :string, doc: "date first observed"
    end
    trusts Teammate
  end
end

The macro produces MyOntology.__ontology__/0 — a payload the Pythonx layer translates into graphiti's entity_types, edge_types, edge_type_map, and excluded_entity_types. The Elixir side never constructs Pydantic classes.

An entity's optional description becomes the extracted type's own description. graphiti's extractor reads it to decide when to mint the entity, so a type whose name alone is ambiguous is extracted far more reliably with one.

Entity and edge type names are unrestricted, but field names are not: graphiti rejects any custom entity attribute colliding with its own EntityNode fields — uuid, name, group_id, labels, created_at, summary, attributes, name_embedding. This DSL does not check that at compile time, so such a field raises EntityTypeValidationError from Python on the first write through the owning Lens.

See test-trees/unit/gralkor-ontology_TEST_TREES.md.