fix(oauth): harden browser launch handling

packages/coding-agent/src/utils/open-browser.ts

@@ -0,0 +1,24 @@
+import { spawn } from "node:child_process";
+
+/**
+ * Open a URL or file in the platform browser/default handler.
+ *
+ * This intentionally never invokes a shell. On Windows, do not use
+ * `cmd /c start`: cmd.exe re-parses metacharacters (&, |, ^, ...) before
+ * `start` runs, which would make attacker-controlled URLs injectable.
+ */
+export function openBrowser(target: string): void {
+	const [cmd, args]: [string, string[]] =
+		process.platform === "darwin"
+			? ["open", [target]]
+			: process.platform === "win32"
+				? ["rundll32", ["url.dll,FileProtocolHandler", target]]
+				: ["xdg-open", [target]];
+
+	// spawn reports launcher failures (for example, missing xdg-open) via an
+	// error event. Browser launch is best-effort: callers still present the target
+	// to the user, so keep the launcher failure from becoming a process crash.
+	spawn(cmd, args, { stdio: "ignore", detached: true })
+		.on("error", () => {})
+		.unref();
+}