fix: add shell flag for Windows spawn in VSCode extension (#2781)

On Windows, npm global installs create .cmd wrapper scripts. Node's
child_process.spawn cannot execute .cmd files directly without
shell: true, resulting in ENOENT/EINVAL errors when the VSCode
extension tries to start the GSD agent.

Closes #2721

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vscode-extension/src/gsd-client.ts

@@ -127,6 +127,7 @@ export class GsdClient implements vscode.Disposable {
 			cwd: this.cwd,
 			stdio: ["pipe", "pipe", "pipe"],
 			env: { ...process.env },
+			shell: process.platform === "win32",
 		});
 		this.process = proc;