Port of the TS packages/pen-acp: ndJSON transport, a hand-rolled
JSON-RPC engine (request-id correlation, session/update notification
routing, session/request_permission auto-approval, and a pending-request
drain on EOF so a dead agent fails fast instead of timing out), and an
AcpConnection driving initialize / session/new / session/prompt over
local stdio or a remote WebSocket. The event adapter maps ACP session
updates onto the chat panel's ChatDelta vocabulary.
