Multi-agent handoffs: verify 'spec is in X' claims before restarting

When handing a multi-agent coding project to a fresh session, a handoff that says a design was 'approved, spec in DOC.md' can be wrong. The agent asked to write the spec may have been stopped before doing it. The new agent then searches the doc, finds nothing, and stalls. Before writing a handoff, grep each referenced doc for the claimed section. Otherwise, write that it is 'approved, not yet written', and include the approved shape inline. The original wording is recoverable from the previous session's transcript JSONL. Grep it with Python's re module: BSD and ugrep choke on long '.{0,N}' context patterns over large single-line JSON.