fix(#466): add detached: true to SessionStart hook spawn for Windows

On Windows, child.unref() alone is insufficient for proper process
detachment. The child process remains in the parent's process group,
causing Claude Code to wait for the hook process tree to exit before
accepting input.

Adding detached: true allows the child process to fully detach on
Windows while maintaining existing behavior on Unix.

Closes #466

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

hooks/gsd-check-update.js

@@ -55,7 +55,8 @@ const child = spawn(process.execPath, ['-e', `
   fs.writeFileSync(cacheFile, JSON.stringify(result));
 `], {
   stdio: 'ignore',
-  windowsHide: true
+  windowsHide: true,
+  detached: true  // Required on Windows for proper process detachment
 });
 
 child.unref();