Fix Windows spawn compatibility for .cmd files

- Add shell: true to spawn options for .cmd file execution on Windows
- Add windowsHide: true to prevent console window from appearing
- Remove detached: true which conflicts with shell: true on Windows

Fixes spawn EINVAL error on Windows when launching opencode.cmd

src/ProcessManager.ts

@@ -85,7 +85,8 @@ export class ProcessManager {
         cwd: this.projectDirectory,
         env: { ...process.env, NODE_USE_SYSTEM_CA: "1" },
         stdio: ["ignore", "pipe", "pipe"],
-        detached: true,
+        shell: true,
+        windowsHide: true,
       }
     );