git-multibranch — same SSH-key contamination as the base pi harness

FAIL in fast2__qwen3.6-35b-a3b__20260707-145458 (x5cwvyt, @3m29s, clean stop, V=19, 16 subagent-lines — the staged workflow ran). Verified identical root cause to the MinimalPi trial: ssh-keygen + append to the git user's authorized_keys (grep hits: keygen 4, authorized_keys 24), so the verifier key-authenticates and its password-expect script aborts before pushing. The reviewer stage did not flag the leftover credential — its checks also went through the agent's own key.

Full analysis: analysis/qwen3.6-35b-a3b/pi/git-multibranch.md.