Search (PQL DSL query) is now the default left tab; Markdown (filtered to .md files) on the right. Clicking a markdown file publishes on the message bus; the markdown extension activates the context panel viewer and re-publishes on a load channel after the frame so the viewer is guaranteed to be mounted. Viewer subscribes to load channel and editor buffer events, publishes focus for sidebar highlighting. Co-Authored-By: Claude <noreply@anthropic.com>