A sibling's PR merging into the parent branch while a dev worked leaves the
dev's branch behind its base — the assembled PR then can't merge cleanly and
the sibling's changes go missing (the 2026-06-27 out-of-order dev-task break).
The behind-base gate refuses i_am_done in that state and steers the dev to
sync_branch (the Phase B1 gate-level rebase verb).
- GitService.is_behind_base: rev-list --left-right --count across
origin/{base}...origin/{head} → (behind, ahead); fetch-first so origin
reflects the pushed head. Raises on git failure (consistent with
rebase_onto_base); malformed stdout degrades to (0,0).
- Choreographer._behind_base_gate: wired into _i_am_done_gate after
_ensure_branch_pushed. behind>0 → invalid_state remediate→sync_branch.
Fail-open on git/base-resolution error (flaky fetch can't strand a task at
the submit gate — the merge layer has its own behind checks). Skipped for
branchless roots and protected bases (master/main/-prefixed).
Tests: gate (6: refuse+steer/up-to-date/branchless/protected/fail-open-base/
fail-open-git), is_behind_base (6: parse/up-to-date/malformed/argv-form/
requires-branch/missing-project). ruff + mypy roboco/ tests/ clean; unit green.
Renn F
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