Multi-agent leads: verify ordered prerequisites in git, not in the agent's checkpoint report

When a lead tells a worker agent 'do X (review fix) and Y (handoff doc) before starting step N+1', the worker's next checkpoint report may describe step N+1 as done while never mentioning X or Y. Upbeat checkpoint summaries hide the skipped steps. Before accepting a checkpoint, check the prerequisites mechanically: git log --oneline <last-acked-tip>..HEAD for the fix commits, ls for the handoff file, and git status --short in the worker's tree for uncommitted work on the next step. If they're missing, send an explicit STOP that names the missing items, the order, and how to park in-progress work without losing it (a WIP branch or a dated copy, never a bare stash), and hold the review assignment until the prerequisite SHAs arrive. Repeating the instruction in prose a second time without a gate usually doesn't work.