feat: make repl the default subcommand

Running `qpq --username alice --password secretA` now launches the
REPL directly instead of requiring `qpq repl ...`. The explicit
`repl` subcommand still works for backwards compatibility.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/quicproquo-client/src/main.rs

@@ -53,8 +53,25 @@ struct Args {
     #[arg(long, global = true, env = "QPQ_STATE_PASSWORD")]
     state_password: Option<String>,
 
+    // ── Default-repl args (used when no subcommand is given) ─────────
+    /// State file path (identity + MLS state). Used when running the default REPL.
+    #[arg(long, default_value = "qpq-state.bin", env = "QPQ_STATE")]
+    state: PathBuf,
+
+    /// Server address (host:port). Used when running the default REPL.
+    #[arg(long, default_value = "127.0.0.1:7000", env = "QPQ_SERVER")]
+    server: String,
+
+    /// OPAQUE username for automatic registration/login.
+    #[arg(long, env = "QPQ_USERNAME")]
+    username: Option<String>,
+
+    /// OPAQUE password (prompted securely if --username is set but --password is not).
+    #[arg(long, env = "QPQ_PASSWORD")]
+    password: Option<String>,
+
     #[command(subcommand)]
-    command: Command,
+    command: Option<Command>,
 }
 
 #[derive(Debug, Subcommand)]
@@ -324,7 +341,7 @@ async fn main() -> anyhow::Result<()> {
 
     // For the REPL, defer init_auth so it can resolve its own token via OPAQUE.
     // For all other subcommands, initialize auth immediately.
-    let is_repl = matches!(args.command, Command::Repl { .. });
+    let is_repl = matches!(args.command, None | Some(Command::Repl { .. }));
     if !is_repl {
         let auth_ctx = ClientAuth::from_parts(args.access_token.clone(), args.device_id.clone());
         init_auth(auth_ctx);
@@ -332,7 +349,15 @@ async fn main() -> anyhow::Result<()> {
 
     let state_pw = args.state_password.as_deref();
 
-    match args.command {
+    // Default to REPL when no subcommand is given.
+    let command = args.command.unwrap_or_else(|| Command::Repl {
+        state: args.state,
+        server: args.server,
+        username: args.username,
+        password: args.password,
+    });
+
+    match command {
         Command::RegisterUser {
             server,
             username,