fix: implement MO_USE_FIND and improve fd fallback logic

Fixes the issue reported in PR #410 where `mo purge` fails to find
artifacts when `fd` returns empty results.

Changes:
- Implement MO_USE_FIND environment variable to force using find
- Improve fd fallback: check if fd output is empty (-s test)
- Add debug logging to show which tool is being used
- If fd returns no results, fallback to find automatically

This fixes the root cause where fd successfully runs (exit 0) but
finds nothing, preventing the find fallback from being triggered.

lib/clean/project.sh

@@ -377,7 +377,11 @@ scan_purge_targets() {
 
     local use_find=true
 
-    if command -v fd > /dev/null 2>&1; then
+    # Allow forcing find via MO_USE_FIND environment variable
+    if [[ "${MO_USE_FIND:-0}" == "1" ]]; then
+        debug "MO_USE_FIND=1: Forcing find instead of fd"
+        use_find=true
+    elif command -v fd > /dev/null 2>&1; then
         # Escape regex special characters in target names for fd patterns
         local escaped_targets=()
         for target in "${PURGE_TARGETS[@]}"; do
@@ -404,12 +408,22 @@ scan_purge_targets() {
         # Try running fd. If it succeeds (exit code 0), use it.
         # If it fails (e.g. bad flag, permissions, binary issue), fallback to find.
         if fd "${fd_args[@]}" "$pattern" "$search_path" 2> /dev/null > "$output_file.raw"; then
-            use_find=false
-            process_scan_results "$output_file.raw"
+            # Check if fd actually found anything - if empty, fallback to find
+            if [[ -s "$output_file.raw" ]]; then
+                debug "Using fd for scanning (found results)"
+                use_find=false
+                process_scan_results "$output_file.raw"
+            else
+                debug "fd returned empty results, falling back to find"
+                rm -f "$output_file.raw"
+            fi
+        else
+            debug "fd command failed, falling back to find"
         fi
     fi
 
     if [[ "$use_find" == "true" ]]; then
+        debug "Using find for scanning"
         # Pruned find avoids descending into heavy directories.
         local prune_dirs=(".git" "Library" ".Trash" "Applications")
         local purge_targets=("${PURGE_TARGETS[@]}")