Add peer/ directory to each agent's 9P namespace. Agents communicate
by writing to peer/{name}, which delivers to the target's prompt handler.
Only declared peers can be messaged — the directory is the ACL.
Implementation:
- Agent struct: peers map + AddPeer/RemovePeer/Peers methods
- fs/spec.go: peer/ Each node (write-only entries), peeradd/peerdel/peers ctl commands
- Bidirectional: peeradd A on B also adds B on A
- Peers constrained to same session
- Persisted with session state (PersistedAgent.Peers field)
- peeradd/peerdel trigger immediate session save
Docs updated: system_prompt.md, AGENTS.md, README.md, architecture-9p.md,
architecture-core.md, architecture.md, usage.md.
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