Improve Enemy Interaction With Environment (Ghosts Ignore Obstacles)

## Description
Ghost enemies currently pass through most obstacles. While this initially feels interesting, it ends up reducing meaningful player movement and removes the ability to use cover or positioning strategically. As a result, many obstacles feel irrelevant from a gameplay perspective.

## Expected Improvement
- Introduce at least some enemies or situations where obstacles matter
- Allow players to use environment features for positioning, avoidance, or crowd control
- Improve "crowd management" so the player must move and make spatial decisions

## Notes
- Ghosts could selectively pass through certain objects
- Mix enemy types: some that respect collision, some that don’t
- Consider temporary barriers or elemental interactions that affect ghosts

Look for enemy AI scripts, likely in a folder like Assets/Scripts/Enemies/. Examine how ghost enemies currently handle collisions, possibly in a GhostEnemy or similar class. The Unity physics system (colliders, layers) will need to be understood to modify which obstacles ghosts ignore. Test changes in a scene with various obstacles to see the gameplay impact.

Written by the indexing model from the issue text.