Contract gate environment repair resolution

This closes the investigation recorded by task task_92acb364fc564112a06112e08df0eb52 for the contract gate that blocked task_8454149df64b4d3f83905115708a4fed.

Resolution

The root cause was Git 2.34.1 on the worker image. The merge gate uses git merge-tree --write-tree, which requires Git 2.38 or newer. The resulting merge-test failures also prevented the serial test slice from running and made the partial coverage total look like an independent coverage regression.

The repair siblings addressed each part of that failure:

• git_prereq makes scripts/run-contract-tests.sh exit before pytest with a distinct, actionable prerequisite error when no Git >= 2.38 is available.
• host_image_git declares Git 2.38 as a repository prerequisite and enforces it in fleet onboarding and worker/container images.
• coverage_margin labels coverage from a failed test run as partial rather than presenting it as a complete gate measurement.

Focused acceptance on Git 2.55.0 passed all 107 tests in tests/test_git_toolchain_floor.py, tests/test_fleet_prerequisite_receipts.py, tests/test_merge_queue.py, tests/test_auto_land.py, and tests/test_predispatch_conflict.py. The staged Git 2.34.1 runner contract confirms exit status 3, an error naming the detected and required versions and the supported override, and no pytest invocation. The parent's absent attempt_hardware_context module is not imported or referenced by the repaired runner or its prerequisite tests; the repair therefore does not depend on the parent deliverable.

The authoritative complete contract gate and its final coverage safety: line remain owned by the deterministic host finalizer for this task.