refactor: Move scenarios to root and update paths

- Move scenarios from app/assets/scenarios/ to scenarios/
- Update Mission model to use BreakEscape::Engine.root instead of Rails.root
- Update seeds.rb to use engine root for scenario discovery
- Update tests to use engine root for path assertions

This ensures scenarios are found correctly in both mounted (Hacktivity)
and standalone (test) environments.

All 12 tests now passing with 19 assertions!

test/models/break_escape/mission_test.rb

@@ -21,7 +21,7 @@ module BreakEscape
 
     test "scenario_path returns correct path" do
       mission = break_escape_missions(:ceo_exfil)
-      expected = Rails.root.join('app', 'assets', 'scenarios', 'ceo_exfil')
+      expected = BreakEscape::Engine.root.join('scenarios', 'ceo_exfil')
       assert_equal expected, mission.scenario_path
     end
   end