Agent Merge is not initiated for all PRs created against different base branches in one session

Testing #334491

## Description
When two pull requests are created from the same Agents session against different base branches, Agent Merge is not initiated for all of them.

## Example
This session created two draft PRs for the same fix:
- #334652: `dileepy/sandbox-policy-1.137` → `release/1.137`
- #334654: `agents/issue-analysis-and-details` → `main`

Agent Merge initiated a CI repair turn for #334652, but did not initiate for #334654. The session's working directory was on the release-specific branch after creating both PRs.

## Steps to reproduce
1. Create a PR targeting a release branch from an Agents session.
2. From the same session, create another PR for the same fix targeting `main`, using a separate head branch.
3. Enable Agent Merge for the session.
4. Observe which PRs Agent Merge monitors and initiates work for.

## Expected
Agent Merge should support both PRs, or provide an explicit way to select and enable it for each PR. If only one PR per session is supported, the UI should clearly identify that limitation and the PR being managed.

## Actual
Agent Merge was initiated for the release PR only. The main PR was not picked up, despite both PRs being created from the same session.

## Separate observation
The repair turn for #334652 could not diagnose its failed macOS / Electron check because the Agent Merge CI tool returned logs truncated before the failure. That is separate from the multiple-PR initiation issue reported here.