Refactor skill allocation to upgrade; implement skill requirements and resource checks; remove experience from Pet model; update README for skill tree progress

Controllers/SkillController.cs

@@ -44,11 +44,11 @@ namespace PetCompanion.Controllers
         }
 
         [HttpPost("{petId}/allocate/{skillId}")]
-        public IActionResult AllocateSkillPoint(string petId, int skillId)
+        public IActionResult UpgradeSkill(string petId, int skillId)
         {
             try
             {
-                var result = _petSkillService.AllocateSkillPoint(petId, userId.ToString(), skillId);
+                var result = _petSkillService.UpgradeSkill(petId, userId.ToString(), skillId);
                 return Ok(result);
             }
             catch (Exception ex)