New runtime plugin registering runtimeId 'acp', mirroring the
fusion-plugin-droid-runtime shape. Adds @agentclientprotocol/sdk@0.24.0
and an SDK smoke-import test that gates on the load-bearing exports
(ClientSideConnection, ndJsonStream, PROTOCOL_VERSION=1) so a breaking
SDK change surfaces at U1. Runtime adapter is a contract-conforming
skeleton (incl. describeModel); session driving lands in U2/U3.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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