ci: optimize GitHub Actions CI/CD workflow

- Remove redundant CNAME creation (L113) as peaceiris/actions-gh-pages
  handles it with cname parameter
- Simplify "Rebuild for smalruby3-gui GitHub Pages" step to only run
  postbuild.mjs instead of full npm run build
- Simplify "Rebuild for branch GitHub Pages" step to only run
  postbuild.mjs instead of full npm run build
- Remove unnecessary NODE_OPTIONS and NODE_ENV environment variables
  from rebuild steps

These changes eliminate duplicate build processes and improve workflow
efficiency since the initial webpack build already completed before
these rebuild steps.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: takaokouji

.github/workflows/ci-cd.yml

@@ -108,9 +108,7 @@ jobs:
                 fi
             - name: Prepare deployment files
               if: github.ref == 'refs/heads/develop' || github.ref == 'refs/heads/master' || github.ref == 'refs/heads/main'
-              run: |
-                touch build/.nojekyll
-                echo "smalruby.app" > build/CNAME
+              run: touch build/.nojekyll
             - name: Deploy playground to Smalruby.app GitHub Pages
               uses: peaceiris/actions-gh-pages@373f7f263a76c20808c831209c920827a82a2847 # v3
               if: github.ref == 'refs/heads/develop' || github.ref == 'refs/heads/master' || github.ref == 'refs/heads/main'
@@ -123,10 +121,8 @@ jobs:
             - name: Rebuild for smalruby3-gui GitHub Pages
               if: github.ref == 'refs/heads/develop' || github.ref == 'refs/heads/master' || github.ref == 'refs/heads/main'
               env:
-                NODE_OPTIONS: --max-old-space-size=4000
-                NODE_ENV: production
                 PUBLIC_PATH: /smalruby3-gui/
-              run: npm run build
+              run: npm exec node ./scripts/postbuild.mjs
             - name: Prepare deployment files for smalruby3-gui GitHub Pages
               if: github.ref == 'refs/heads/develop' || github.ref == 'refs/heads/master' || github.ref == 'refs/heads/main'
               run: touch build/.nojekyll
@@ -151,10 +147,8 @@ jobs:
                   startsWith(github.ref, 'refs/heads/renovate/')
                 ))
               env:
-                NODE_OPTIONS: --max-old-space-size=4000
-                NODE_ENV: production
                 PUBLIC_PATH: /smalruby3-gui/${{ steps.branch.outputs.BRANCH_NAME }}/
-              run: npm run build
+              run: npm exec node ./scripts/postbuild.mjs
             - name: Prepare deployment files for branch GitHub Pages
               if: |
                 (!(