fix(review): isolate review sub-task from getRunningTasks()

[robotlearning123]

Context

PR #37 added cross-agent diff review. The review agent is spawned via this.run(reviewTask, "/tmp"), which inserts the review task into _runningTasks. During review (~60s), API callers see a ghost task in getRunningTasks() that can't be resolved by getTask().

Options

1. Add a _isReviewTask flag and filter in getRunningTasks()
2. Use a separate AgentRunner instance for reviews
3. Accept the brief visibility (document it)

Impact

Low — the ghost task disappears after review completes. But it's confusing for dashboard consumers.

Ref: #37