feat(#1339): Deploy Nexus to proper URL for preview

ES module imports fail via file:// or raw Forge URLs (CORS +
missing Content-Type headers). boot.js detects this and warns users.

Three deployment options:
  - ./preview.sh — local Python server, correct MIME types
  - docker compose up nexus-preview — nginx on :8080 + WS proxy
  - Push to main — GitHub Pages auto-deploy

Fixes #1339

.github/workflows/pages.yml

@@ -0,0 +1,35 @@
+name: Deploy Nexus Preview to Pages
+on:
+  push:
+    branches: [main]
+  workflow_dispatch:
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+concurrency:
+  group: "pages"
+  cancel-in-progress: false
+jobs:
+  deploy:
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/configure-pages@v5
+      - name: Prepare static assets
+        run: |
+          mkdir -p _site
+          cp index.html app.js style.css boot.js gofai_worker.js _site/
+          cp service-worker.js manifest.json robots.txt help.html _site/
+          cp portals.json vision.json _site/
+          cp -r nexus/ _site/nexus/
+          cp -r icons/ _site/icons/ 2>/dev/null || true
+          cp -r assets/ _site/assets/ 2>/dev/null || true
+      - uses: actions/upload-pages-artifact@v3
+        with:
+          path: '_site'
+      - id: deployment
+        uses: actions/deploy-pages@v4