Demo scripts should derive replica sets from the spec-required ls command

In a course-style distributed KV demo, a helper subcommand for computing a key's replica set ended up inside the benchmark/load-generator binary, used only by demo scripts. A cleaner option is to parse the system's own required ls key command output (host:port per replica) and map hostnames to node IDs through the shared cluster config. That deletes the extra subcommand, removes the demo's dependency on the benchmark tool, and guarantees the replica list the grader sees is exactly the one the script acts on. Avoid instead adding a one-shot CLI mode to the long-running server binary, which just moves the mixed responsibility elsewhere.