Task: Add variability guards: introduce #+sql_only and #+has_qt flags to the entity schema

SQL-only and Qt-less entities are expressible in ores.codegen.entity format without errors from missing C++ or Qt sections.

• #+sql_only: true suppresses C++ and Qt profile dispatch for the tagged entity.
• #+has_qt: false suppresses Qt profile dispatch while leaving C++ profiles active.
• generator.py raises a clear error if C++ or Qt sections are present on a sql_only entity.
• purpose_type entity file is syntactically valid with #+sql_only: true.

(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.)