fix: add fallback strategy for missing gh-pages branch

- Add validation for current-site content availability
- Implement fallback from preserve-base-site to merge-and-build when no base site exists
- Resolve issue where missing gh-pages branch caused workflow failures
- Enable graceful degradation for incremental builds when base site unavailable

Root cause: GitHub Pages deployed via artifacts (not gh-pages branch)
Solution: Auto-fallback to full rebuild when current-site empty/missing

Fixes incremental federation builds when no previous site state available.

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

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

Author: InfoTech.io Bot

.github/workflows/deploy-github-pages.yml

@@ -57,11 +57,12 @@ jobs:
           path: current-site
         continue-on-error: true
 
-      - name: Validate Current State
+      - name: Validate Current State & Apply Fallback
         run: |
           echo "📦 Checking current site state..."
 
-          if [ -d "current-site" ]; then
+          # Check if we have valid current site content
+          if [ -d "current-site" ] && [ -n "$(ls -A current-site 2>/dev/null)" ]; then
             echo "✅ Current site downloaded"
             echo "📊 Current site size: $(du -sh current-site | cut -f1)"
 
@@ -72,9 +73,16 @@ jobs:
             if [ -d "current-site/docs" ]; then
               echo "✅ Docs directory exists: $(du -sh current-site/docs | cut -f1)"
             fi
+
+            # Valid current site found
+            echo "FALLBACK_TO_FULL=false" >> $GITHUB_ENV
           else
-            echo "⚠️  No existing site found (fresh deployment)"
+            echo "⚠️  No existing site found (fresh deployment or gh-pages branch missing)"
+            echo "🔄 Will use full rebuild strategy as fallback"
             mkdir -p current-site
+
+            # Set fallback flag
+            echo "FALLBACK_TO_FULL=true" >> $GITHUB_ENV
           fi
 
       # ==========================================
@@ -150,6 +158,15 @@ jobs:
             exit 1
           fi
 
+          # CRITICAL FIX: Apply fallback strategy if base site unavailable
+          if [ "$STRATEGY" = "preserve-base-site" ] && [ "$FALLBACK_TO_FULL" = "true" ]; then
+            echo "⚠️  Base site unavailable - switching to full rebuild strategy"
+            echo "🔄 Fallback: preserve-base-site → merge-and-build"
+            STRATEGY="merge-and-build"
+            BUILD_CORPORATE="true"  # Must rebuild corporate site too
+            BUILD_DOCS="true"
+          fi
+
           echo "strategy=$STRATEGY" >> $GITHUB_OUTPUT
           echo "build_corporate=$BUILD_CORPORATE" >> $GITHUB_OUTPUT
           echo "build_docs=$BUILD_DOCS" >> $GITHUB_OUTPUT