fix(mosaic): stop yolo runtime from leaking runtime name as first user message (#455)

Fixes mosaicstack/stack#454

Co-authored-by: Jason Woltje <jason@diversecanvas.com>
Co-committed-by: Jason Woltje <jason@diversecanvas.com>

packages/mosaic/src/commands/launch.ts

@@ -757,8 +757,23 @@ function runUpgrade(args: string[]): never {
 
 // ─── Commander registration ─────────────────────────────────────────────────
 
-export function registerLaunchCommands(program: Command): void {
-  // Runtime launchers
+/**
+ * Handler invoked when a runtime subcommand (`<runtime>` or `yolo <runtime>`)
+ * is parsed. Exposed so tests can exercise the commander wiring without
+ * spawning subprocesses.
+ */
+export type RuntimeLaunchHandler = (
+  runtime: RuntimeName,
+  extraArgs: string[],
+  yolo: boolean,
+) => void;
+
+/**
+ * Wire `<runtime>` and `yolo <runtime>` subcommands onto `program` using a
+ * pluggable launch handler. Separated from `registerLaunchCommands` so tests
+ * can inject a spy and verify argument forwarding.
+ */
+export function registerRuntimeLaunchers(program: Command, handler: RuntimeLaunchHandler): void {
   for (const runtime of ['claude', 'codex', 'opencode', 'pi'] as const) {
     program
       .command(runtime)
@@ -766,11 +781,10 @@ export function registerLaunchCommands(program: Command): void {
       .allowUnknownOption(true)
       .allowExcessArguments(true)
       .action((_opts: unknown, cmd: Command) => {
-        launchRuntime(runtime, cmd.args, false);
+        handler(runtime, cmd.args, false);
       });
   }
 
-  // Yolo mode
   program
     .command('yolo <runtime>')
     .description('Launch a runtime in dangerous-permissions mode (claude|codex|opencode|pi)')
@@ -784,8 +798,21 @@ export function registerLaunchCommands(program: Command): void {
         );
         process.exit(1);
       }
-      launchRuntime(runtime as RuntimeName, cmd.args, true);
+      // Commander includes declared positional arguments (`<runtime>`) in
+      // `cmd.args` alongside any trailing excess args. Slice off the first
+      // element so we forward only true excess args — otherwise the runtime
+      // name leaks into the underlying CLI as an initial positional arg,
+      // which Claude Code interprets as the first user message.
+      // Regression test: launch.spec.ts, issue mosaicstack/stack#454.
+      handler(runtime as RuntimeName, cmd.args.slice(1), true);
     });
+}
+
+export function registerLaunchCommands(program: Command): void {
+  // Runtime launchers + yolo mode wired to the real process-replacing launcher.
+  registerRuntimeLaunchers(program, (runtime, extraArgs, yolo) => {
+    launchRuntime(runtime, extraArgs, yolo);
+  });
 
   // Coord (mission orchestrator)
   program