Adds CompletableFuture-based request-reply mechanism for commands that
need synchronous results. CommandReply record in core, pendingReplies
map in AgentRegistryService, test-expression endpoint on config controller
with 5s timeout. CommandAckRequest extended with optional data field.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>