#414 Long-held transactions during task execution cause connection pool pressure

## Problem During test execution, observed up to 34 idle_in_transaction connections with total connections growing from 50 to 115. ## Root Cause engine/orchestrator.py:370-447 holds transactions open for entire task execution. ## Evidence - step_succeeded avg duration: 20.1 seconds - step_succeeded max duration: 68.1 seconds - Peak idle_in_transaction: 34 connections ## Notes This is INTENTIONAL for atomicity. The tradeoff is correctness vs scalability.