Fix skill study mana deduction in skillStore.ts

- Added useManaStore import to skillStore.ts
- Added mana deduction logic in startStudyingSkill action
- Mana is now properly deducted when starting to study a skill (unless already paid)

AGENTS.md

@@ -560,6 +560,10 @@ The following mana types have been **removed** and should **never be re-added**:
 - `mental` - Mind/psionic themed (removed for design consistency)
 - `force` - Telekinetic themed (removed for design consistency)
 
+### Removed Features
+- LabTab — permanently removed. Do not re-add under any name.
+  The lab element conversion UI was replaced by attunements.
+
 ---
 
 ## 🔮 Mana Types Overview