fetch_schema rewrites src/osw/model/entity.py as a side effect of ordinary read/write calls

[LukasGold]

fetch_schema regenerates src/osw/model/entity.py inside the installed package. It is not called explicitly by the user, it is reached transitively from ordinary operations, currently create_or_update_entity and export_entity_jsonld in the MCP server (#133), and soon from the planned CLI.

Why it matters

• Under a normal pip install osw this writes into site-packages: wasteful and surprising, but contained.
• In an editable/source checkout it modifies a tracked file in the developer's working tree. Running a tool that reads or writes one entity produces an unrelated diff in git status.
• The regenerated output is not always valid, see fetch_schema generates syntactically invalid osw/model/entity.py (RiskAssessmentProcess default_factory -> SyntaxError) #125.

Suggested direction

• Make the write opt-in (explicit fetch_schema call or a flag), and have the transitive callers use an in-memory or cache-dir model instead.
• Failing that, write to a user cache dir rather than the package directory.

Interim mitigation in the MCP/CLI layer: document the side effect in the help text of the affected commands.