Bounded RPC message_update stream; loud stdout failures
- Breaking change for RPC consumers:
message_update events no longer include the cumulative message or the assistantMessageEvent.partial snapshot. Rebuild the message from the deltas, and use message_end for the final message. This applies to plain rpc, agent/subagent, and dashboard modes, and stops per-token frames from growing with message length.
- New
--rpc-full-message-updates flag (requires --mode rpc; not available with --ui dashboard) restores the old full frames. It is forwarded to subagent children.
- If writing to stdout fails (e.g. EPIPE), dreb now prints a clear diagnostic and exits nonzero instead of crashing on an unhandled error. Once the failure is detected, an RPC shutdown can no longer override it with exit 0.
- New wire types
RpcMessageUpdateEvent / RpcSessionEvent.
Resolves issue 535 via PR 544.
