Grok emits `_x.ai/session_notification` / `_x.ai/session/update` for
hook_execution status. The ACP SDK routes those to Client.extNotification;
without it, every successful post_tool_use hook logged -32601 Method not
found. Implement no-op extMethod/extNotification on default and bridging
handlers in acp-runtime and the Grok vendored copy.