Task cancellation left the task's PR open on the forge forever: cancel()
now best-effort-closes the recorded PR for the task and its cascaded
descendants (close_task_pr_best_effort resolves owner/repo off git_url —
no clone needed; never raises into the cancel). The bulk stale-branch
sweep gains a dependents guard: a branch still recorded by a non-terminal
task, or serving as a live child's resolve_parent_branch base, is excluded
from the candidate window — mirroring the existing env-ladder-rung skip.
Scoped to the sweep, not delete_task_branch, so the BFS cascade-cancel
can't falsely block a parent's branch on its own about-to-cancel child.
Co-authored-by: Renn F <rennf93@users.noreply.github.com>
