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/Cargo.toml

@@ -74,6 +74,11 @@ crossterm = { version = "0.28", optional = true }
 # YAML playbook parsing (only compiled with --features playbook).
 serde_yaml = { version = "0.9", optional = true }
 
+# v2 SDK-based CLI (thin shell over quicproquo-sdk).
+quicproquo-sdk = { path = "../quicproquo-sdk", optional = true }
+quicproquo-rpc = { path = "../quicproquo-rpc", optional = true }
+rustyline      = { workspace = true, optional = true }
+
 [lints]
 workspace = true
 
@@ -86,6 +91,8 @@ tui = ["dep:ratatui", "dep:crossterm"]
 # Enable playbook (scripted command execution): YAML parser + serde derives.
 # Build: cargo build -p quicproquo-client --features playbook
 playbook = ["dep:serde_yaml"]
+# v2 CLI over SDK: cargo build -p quicproquo-client --features v2
+v2 = ["dep:quicproquo-sdk", "dep:quicproquo-rpc", "dep:rustyline"]
 
 [dev-dependencies]
 dashmap       = { workspace = true }