Cross-session messaging for DSH

Key features

• Any session on the machine can list and message any other — Claude Code-style horizontal messaging, no daemon.
• list_agents gains scopepeers (other live sessions on this machine: name, status, cwd, last activity) and scope: all (descendants + peers).
• The stock children/descendants scopes are delegated to the captured stock definition.
• send_message accepts a peer name or ref in to alongside subagent ids, with an optional summary (5–10 word recap shown in the target UI).
• The stock tools stay registered