fix(hooks): execFileSync 'npm' needs shell:true on Windows

Without shell:true, execFileSync('npm', ...) on Windows fails with
ENOENT because npm is distributed as npm.cmd, not as a literal 'npm'
binary. The silent try/catch swallows the error, latest stays null,
update_available becomes null, and the statusline never shows
"⬆ /gsd-update" — Windows users miss every release.

Adding shell:true makes execFileSync route through cmd.exe which
resolves npm.cmd via PATHEXT, identical behavior on POSIX.

Repro on Windows:
  $env:GSD_CACHE_FILE = "$env:USERPROFILE\.cache\gsd\gsd-update-check.json"
  node ~\.claude\hooks\gsd-check-update-worker.js
  Get-Content "$env:USERPROFILE\.cache\gsd\gsd-update-check.json"

Before: {"update_available":null,"installed":"1.40.0","latest":"unknown",...}
After:  {"update_available":false,"installed":"1.40.0","latest":"1.40.0",...}

hooks/gsd-check-update-worker.js

@@ -93,6 +93,11 @@ try {
     encoding: 'utf8',
     timeout: 10000,
     windowsHide: true,
+    // shell:true is required on Windows so 'npm' resolves to npm.cmd via PATHEXT.
+    // Without it, execFileSync looks for a literal 'npm' binary, fails with ENOENT,
+    // the catch swallows the error, latest stays null, and the statusline never shows
+    // the "⬆ /gsd-update" indicator on Windows.
+    shell: true,
   }).trim();
 } catch (e) {}