feat(client): v2 CLI binary over SDK with simplified commands

Add v2_main.rs and v2_commands.rs as thin wrappers over QpqClient from
quicproquo-sdk. Activated via --features v2 (feature-gated so v1 main.rs
remains the default). Includes auto-server-launch, clap-based command
surface (register-user, login, whoami, health, resolve, dm, group,
devices), and ClientConfig-based connection setup. MLS-dependent commands
(send, recv, group create/invite) print stubs pointing to the REPL.

crates/quicproquo-client/src/lib.rs

@@ -20,6 +20,8 @@ use std::sync::atomic::{AtomicBool, Ordering};
 use zeroize::Zeroizing;
 
 pub mod client;
+#[cfg(feature = "v2")]
+pub mod v2_commands;
 
 pub use client::commands::{
     cmd_chat, cmd_check_key, cmd_create_group, cmd_demo_group, cmd_export, cmd_export_verify,