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/client/v2_repl.rs

@@ -0,0 +1,3 @@
+//! v2 REPL — interactive mode over the SDK.
+//!
+//! Placeholder module; full implementation will replace the v1 repl.