Adds a file-group-reviewer subagent and wires it into /pr-preflight:preflight: for a diff large/varied enough that direct review would go shallow, it fans out one subagent per file group instead. Verified the dispatch mechanism works in isolation; in practice it rarely triggers since Claude's context window handles direct review well past the stated size thresholds — those are a safety valve, not a hard rule (see TESTING.md for the full account).