fix: suppress reporter noise in test output

- Move "HTML report:" message from finalize to at_exit hook (only
  prints when global reporter has failures, not in unit tests)
- Gate "Reporter skipped" warn behind ENV["DEBUG"]
- Remove verbose error backtrace from at_exit (noise in test output)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: pftg

lib/capybara_screenshot_diff/reporters/html.rb

@@ -28,7 +28,7 @@ def record(assertions)
 
           @failures << failure_entry_for(assertion.name, compare)
         rescue => e
-          warn "[snap_diff] Reporter skipped '#{assertion.name}': #{e.message}"
+          warn "[snap_diff] Reporter skipped '#{assertion.name}': #{e.message}" if ENV["DEBUG"]
         end
       end
 
@@ -37,7 +37,6 @@ def finalize
 
         @finalized = true
         write_report
-        $stdout.puts "[snap_diff] HTML report: #{output_path}"
       end
 
       def passed = total - failures.size
@@ -97,8 +96,8 @@ def write_report
 at_exit do
   CapybaraScreenshotDiff.reporters.each do |reporter|
     reporter.finalize
+    $stdout.puts "[snap_diff] HTML report: #{reporter.output_path}" if reporter.is_a?(CapybaraScreenshotDiff::Reporters::HTML) && !reporter.failures.empty?
   rescue => e
     warn "[snap_diff] Reporter #{reporter.class} failed (#{e.class}: #{e.message})"
-    warn e.full_message(highlight: false) if e.respond_to?(:full_message)
   end
 end