Print cache hit/miss status in HPC build cache setup

Reports whether existing build artifacts were found in the persistent
cache directory, along with the last build timestamp. Helps diagnose
whether Phoenix and Frontier runners are getting cache hits.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sbryngelson

.github/scripts/setup-build-cache.sh

@@ -36,4 +36,13 @@ fi
 ln -s "$_cache_dir" "build"
 
 echo "  Symlink: build -> $_cache_dir"
+
+# Report cache hit/miss based on whether compiled artifacts exist
+_cache_sim=$(find "$_cache_dir" -name 'simulation' -type f 2>/dev/null | head -1)
+if [ -n "$_cache_sim" ]; then
+    _cache_age=$(stat -c '%y' "$_cache_sim" 2>/dev/null || stat -f '%Sm' "$_cache_sim" 2>/dev/null || echo "unknown")
+    echo "  Cache status: HIT (found existing build artifacts, last built: $_cache_age)"
+else
+    echo "  Cache status: MISS (no existing build artifacts)"
+fi
 echo "========================="