POST /api/agent/v1/execute

Execute an MBQL query and return results.

Accepts a base64-encoded MBQL query (as returned by /v1/construct-query) and executes it, returning results with column metadata.

Response format:

• On success: {:data {:cols [...] :rows [...]} :row_count N :status :completed :running_time M}
• On failure: {:status :failed :error "message" ...}

Standard userspace query limits are enforced (2000 rows for simple queries, 10000 for aggregated).