fix: show help menu when running lux with no arguments

Previously `lux` with no args entered the REPL. Now it shows the help
menu. Use `lux repl` to start the REPL explicitly.

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

src/main.rs

@@ -205,6 +205,10 @@ fn main() {
                     compile_to_c(&args[2], output_path, run_after, emit_c);
                 }
             }
+            "repl" => {
+                // Start REPL
+                run_repl();
+            }
             "doc" => {
                 // Generate API documentation
                 generate_docs(&args[2..]);
@@ -214,7 +218,7 @@ fn main() {
                 if !std::path::Path::new(cmd).exists() && !cmd.starts_with('-') && !cmd.contains('.') && !cmd.contains('/') {
                     let known_commands = vec![
                         "fmt", "lint", "test", "watch", "init", "check", "debug",
-                        "pkg", "registry", "serve", "compile", "doc",
+                        "pkg", "registry", "serve", "compile", "doc", "repl",
                     ];
                     let suggestions = diagnostics::find_similar_names(cmd, known_commands.into_iter(), 2);
                     if !suggestions.is_empty() {
@@ -229,8 +233,8 @@ fn main() {
             }
         }
     } else {
-        // Start REPL
+        // No arguments — show help
-        run_repl();
+        print_help();
     }
 }
 
@@ -240,7 +244,8 @@ fn print_help() {
     println!();
     println!("{}", bc("", "Usage:"));
     println!();
-    println!("  {}                        Start the REPL", bc(colors::CYAN, "lux"));
+    println!("  {}                        Show this help", bc(colors::CYAN, "lux"));
+    println!("  {}                   Start the REPL", bc(colors::CYAN, "lux repl"));
     println!("  {} {}             Run a file (interpreter)", bc(colors::CYAN, "lux"), c(colors::YELLOW, "<file.lux>"));
     println!("  {} {} {}   Compile to native binary", bc(colors::CYAN, "lux"), bc(colors::CYAN, "compile"), c(colors::YELLOW, "<file.lux>"));
     println!("  {} {} {} {}   Compile with output name", bc(colors::CYAN, "lux"), bc(colors::CYAN, "compile"), c(colors::YELLOW, "<f>"), c(colors::YELLOW, "-o app"));