Turns the stack map on the pull request page into a working control surface. - **Rebase the stack** starts the durable restack operation and reports queued, running, paused-on-conflict, finished, failed, and cancelled states with a **Check progress** action. - **Remove from stack** unstacks the top layer and returns the page to a standalone review. - The trunk row links to the repository tree at the trunk ref. - A readiness line states how many layers are ready to merge bottom-first. - Actions render only for writers; readers keep the map, the readiness line, and the trunk link. Closes #114