open_pr -> GitService.create_pr posted "base": parent straight to GitHub, so
when the parent (an ancestor task's integration branch) was never pushed — a PM
paused before its first push, or the workspace was wiped — GitHub 422'd "base
field invalid" and stranded every child PR. The base-existence fallback added
in 3d9dd298 lived only in create_pull_request, which open_pr never calls.
Add _ensure_base_on_remote and call it in create_pr: if the base branch is
absent on origin, create it off the default branch's tip (preserving the
integration hierarchy) instead of failing; fall back to the default branch only
if that create push itself fails. Covered by 3 new tests.
Renn F
3d8c0e1c54