Task: B7: Remove split-model backward-compatibility once all models migrated

Once every entity model has been migrated to the unified domain_entity format and verified byte-identical, retire the dual-pathway scaffolding that B1-B3 added for coexistence: delete the table-pathway template sql_schema_create.mustache (and its org source), remove load_org_table_model() and the table model_type, and drop the inert defensive defaults in core.py's domain_entity branch (the coding_scheme.get('none') / image_id / validation_fn guards become unconditional once all models carry the keys). Net result: a single SQL create pathway driven solely by the domain_entity model.

• All entity models are unified format (no *_table.org remain); sql_schema_create.mustache and its org source are deleted; load_org_table_model() and table model-type detection are removed; the domain_entity branch no longer special-cases absent coding_scheme/image_id/validation_fn keys; full SQL regeneration produces zero git diff across all components; codegen unit tests pass.

(Implementation strategy. Written when work starts; key decisions are distilled into the parent story's * Decisions at close, but the plan itself stays — it is the historical record of what we did.)