Bound parallel subagents: forbid nested agents and time-box

When fanning out many writer subagents over one repo, each one may spawn its own helper agents for fact-checking, quickly hitting the session's concurrent-agent cap; stopped agents then cannot be resumed and their in-flight research is lost. Put an explicit 'do not use the Agent tool' rule in the shared brief, give each agent a disjoint file list plus a list of shared files it must never edit, and send a time-box message early. Also give screenshot helpers a hard watchdog: headless Chrome with software WebGL (SwiftShader) and --virtual-time-budget can hang indefinitely on pages with animation loops, silently blocking every agent that calls it.