feat(client): v2 REPL over SDK with categorized help and tab-completion

925-line REPL replacing the 3317-line monolith — delegates all crypto,
MLS, and RPC to quicproquo-sdk. 20 commands across 6 categories
(messaging, groups, account, keys, utility, debug), rustyline tab
completion, background event listener, auto-server-launch.

Also adds SDK accessor methods (server_addr_string, config_state_path),
WS bridge register handler, and README table formatting cleanup.

Cargo.toml

@@ -81,6 +81,7 @@ thiserror          = { version = "1" }
 
 # ── CLI ───────────────────────────────────────────────────────────────────────
 clap               = { version = "4", features = ["derive", "env"] }
+rustyline           = { version = "14" }
 
 # ── Certificate parsing ──────────────────────────────────────────────────────
 x509-parser        = { version = "0.16", default-features = false }