Sort completed background jobs by last interaction time, not session duration

Problem

In the background jobs list (/jobs or the jobs panel), completed tasks appear to be sorted by session duration. This makes it hard to find recently finished work — a short task that just completed gets buried below older long-running sessions.

Proposed Change

Sort completed background jobs by last interaction timestamp (descending — most recent first) instead of session duration.

This matches the mental model of "what just finished?" which is the most common reason to check the completed list.

Alternatives Considered

• A configurable sort option (by duration, by start time, by end time) would be even better, but sorting by last interaction as the default is the minimum viable improvement.