Verified the ACP entrypoint against a real install (grok 0.2.11): the
command is `grok agent stdio`, and an initialize handshake returns a
valid ACP response (protocolVersion 1, model grok-build). Running
prompts requires a SuperGrok or X Premium+ login.
Adds Kiro CLI as an opt-in ACP provider catalog entry, plus a generic ACP extension-notification sink so unknown notifications no longer fail JSON-RPC dispatch.