feat(adapters): long-running channel workers that wake sessions

[justrach]

exo's adapter subsystem: host-managed worker processes (JSONL over stdin/stdout, config via env), an outbox, and inbound events waking a conversation as a normal turn. Their workers cover IRC/WhatsApp/Signal/Discord.

For graff the near-term fit is Discord/Slack fronts for @codegraff-bot rather than a general chat matrix; the durable part worth copying is the worker protocol + outbox + wake seam, which the cube/serve session layer is already close to. Lower priority than sandbox/tools/schedule.

[justrach]

Shipped on #649. Protocol + outbox + wake only: inbound {"type":"message","text"} wakes the session; /adapter send writes {"type":"send","text"}. Discord/Slack fronts are out of scope.