Fix josh-proxy rejecting stored filter path with slash

Josh-proxy's parser treats "/" in :+ paths as a filter separator,
so :+.josh-filters/backend fails. Use flat naming at repo root:
.josh-filter-<target>.josh referenced as :+.josh-filter-<target>.

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

bin/josh-sync

@@ -431,7 +431,7 @@ cmd_preflight() {
     echo "4. Exclude filter files"
 
     while IFS= read -r target_name; do
-      local filter_file=".josh-filters/${target_name}.josh"
+      local filter_file=".josh-filter-${target_name}.josh"
       if [ -f "$filter_file" ]; then
         if git ls-files --error-unmatch "$filter_file" >/dev/null 2>&1; then
           pass "${filter_file} exists and is tracked"