orch-monitor shows status:pr_open even after PR is merged

[proboscis]

Problem

When a run's PR is merged, the status remains pr_open instead of transitioning to done.

Expected Behavior

After a PR is merged:

1. The run status should change from pr_open → done
2. The MERGED column should show merged (not dirty)

Current Behavior

• Status stays at pr_open
• MERGED column may show dirty even for merged PRs

Reproduction

1. Start a run: orch run <issue>
2. Wait for agent to create PR (status: pr_open)
3. Merge the PR on GitHub
4. Run orch ps - status still shows pr_open

Likely Cause

The daemon's PR merge detection may not be polling correctly, or the state transition logic isn't triggering on merge events.