Any Claude Code session can message any other (not just Agent-Teams teammates),
no tmux. Append-only jsonl bus + UserPromptSubmit hook pulling unread per turn.
kei-message.sh (send/inbox/list, address by cwd-basename or "all"),
mailbox-inject.sh (cursor dedup, first-turn baseline, no self-echo), bin/kei
`message` dispatch, lib-scaffold copies all scripts/*.sh, snippet wires the hook.
Bypass KEI_MAILBOX_BYPASS=1. Verified by 2-session simulation.
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
