fix: resolve mapfile compatibility issue in build architecture guard

- Replace mapfile command with portable alternative for cross-shell compatibility
- Add troubleshooting documentation for common shell compatibility issues
- Update BUILDING.md with Build Architecture Guard documentation
- Ensure script works across different shell environments

Fixes pre-commit hook failures on macOS and other systems where mapfile is not available.

scripts/build-arch-guard.sh

@@ -111,7 +111,10 @@ main() {
     log_info "Running Build Architecture Guard..."
     
     # Collect changed files
-    mapfile -t changed_files < <(collect_files "$mode" "$arg")
+    changed_files=()
+    while IFS= read -r line; do
+        [[ -n "$line" ]] && changed_files+=("$line")
+    done < <(collect_files "$mode" "$arg")
     
     if [[ ${#changed_files[@]} -eq 0 ]]; then
         log_info "No files changed, guard check passed"