Refactor inventory unequip logic to use item ID instead of equip target; update README for inventory and skill tree progress

Repositories/PetSkillRepository.cs

@@ -13,6 +13,13 @@ namespace PetCompanion.Repositories
             _context = context;
         }
 
+        public Skill GetSkill(int id)
+        {
+            return _context.Skills
+                .Include(s => s.Effects)
+                .FirstOrDefault(s => s.Id == id);
+        }
+
         public IEnumerable<PetSkill> GetPetSkills(string petId)
         {
             return _context.PetSkills