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

Services/PetSkillService.cs

@@ -32,6 +32,7 @@ namespace PetCompanion.Services
             if (pet.SkillPoints <= 0)
                 throw new Exception("No skill points available");
 
+            var skill = _petSkillRepository.GetSkill(skillId);
             var skills = _petSkillRepository.GetPetSkills(petId);
             var existingSkill = skills.FirstOrDefault(s => s.SkillId == skillId);
 
@@ -45,6 +46,11 @@ namespace PetCompanion.Services
             }
             else
             {
+                if (!skill.SkillsIdRequired.TrueForAll(ni => pet.Skills.Any(s => s.SkillId == ni)))
+                {
+                    throw new Exception("Missing required skill");
+                }
+
                 existingSkill = new PetSkill
                 {
                     PetId = petId,