refactor assigner using DefinedSimpleRelationship

[matt-beanland]

Motivation

AssignedToRelationship stores pool, thing, assigned as direct attributes — a one-off resource that conflates the assignment policy with the relationship primitive. With DefinedRelationship (#140) in place this can be cleaned up.\n\n## Proposed change\n\nUpdate Assigner to create DefinedRelationship records with type: :assigned_to and the relevant embedded characteristics (pool, thing, assigned value). Remove AssignedToRelationship. Update FreeValues calculation to query DefinedRelationship.\n\n## Dependencies\n\nRequires #140 (DefinedRelationship) to be merged first.

[matt-beanland]

what was done:
assigner.ex — creates DefinedSimpleRelationship with type: :assignedTo and characteristic: %NameValuePrimitive{name: thing, value: Primitive.wrap("integer", value)}. find_assignment queries by source_id + target_id + type and Elixir-filters by characteristic.name/value.
free_values.ex / assigned_values.ex — query DefinedSimpleRelationship by source_id + type: :assignedTo, filter in Elixir by characteristic.name == thing.
base_instance.ex — assignments has_many and Neo4j wiring switched to DefinedSimpleRelationship.
assigner_test.exs — hd(card.assignments).assigned → Diffo.Unwrap.unwrap(hd(card.assignments).characteristic.value).
AssignedToRelationship — deleted, removed from domain.