fix: skip web build on Windows — Next.js webpack hits EPERM on system dirs

The Windows CI runner's Application Data directory triggers EPERM
when webpack scans it. The web build is a Linux/macOS deployment
target and doesn't need to run on Windows.

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

scripts/build-web-if-stale.cjs

@@ -17,6 +17,12 @@ const { execSync } = require('node:child_process')
 const { existsSync, readdirSync, statSync } = require('node:fs')
 const { join, resolve } = require('node:path')
 
+// Skip on Windows — Next.js webpack build hits EPERM scanning system dirs
+if (process.platform === 'win32') {
+  console.log('[gsd] Web build skipped on Windows.')
+  process.exit(0)
+}
+
 const root = resolve(__dirname, '..')
 const webRoot = join(root, 'web')
 // Also watch src/ because api routes import directly from src/web/* and src/resources/*