TDD step 4: read_sql executes one read-only SELECT against the sqlite
database whose path is resolved at run time from the env var named by
connection_ref (credentials never in the manifest). Read-only enforced
by the connection mode (file:...?mode=ro), single statement via
Connection.execute, §5 cell text conversion, streaming max_rows cap,
and every sqlite failure wrapped in SourceError.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QeqhJpYQyghASjiJo5EhGg
