fix(fs-search): respect platform path separators

Group paths using node:path.sep so POSIX backslashes remain filename characters while Windows continues to treat them as separators.

examples/acp-agent/tests/fixtures/fs-search-bin/rg

@@ -3,6 +3,8 @@ printf '%s\n' \
   'archive/a.ts' \
   'archive/b.ts' \
   'archive/c.ts' \
+  'old\one' \
+  'old\two' \
   'src/index.ts' \
   'docs/guide.md' \
   'test/spec.ts'