Update documentation after refactoring: AGENTS.md, GAME_BRIEFING.md, skills.md

- Updated AGENTS.md to include store/ directory and clarify store architecture
- Updated GAME_BRIEFING.md Code Architecture section with store/ and legacy store info
- Updated skills.md with skill state management information
- Includes other refactoring changes (store hooks, component updates, etc.)

src/components/game/SpellsTab.tsx

@@ -1,6 +1,6 @@
 'use client';
 
-import { useGameStore, canAffordSpellCost } from '@/lib/game/store';
+import { useGameStore, canAffordSpellCost, fmt } from '@/lib/game/stores';
 import { ELEMENTS, SPELLS_DEF } from '@/lib/game/constants';
 import { useStudyStats } from '@/lib/game/hooks/useGameDerived';
 import { Card, CardContent, CardHeader, CardTitle } from '@/components/ui/card';
@@ -51,7 +51,7 @@ export function SpellsTab() {
             <h3 className={`text-lg font-semibold mb-3 ${tierColors[tier]}`}>{tierNames[tier]}</h3>
             <div className="grid grid-cols-1 md:grid-cols-2 lg:grid-cols-3 gap-4">
               {spellsInTier.map(([id, def]) => {
-                const state = store.spells[id];
+                const state = store.spells?.[id];
                 const learned = state?.learned;
                 const isStudying = store.currentStudyTarget?.id === id;
                 const elemDef = def.elem === 'raw' ? null : ELEMENTS[def.elem];
@@ -147,7 +147,7 @@ export function SpellsTab() {
                             variant={canStudy ? 'default' : 'outline'}
                             disabled={!canStudy}
                             className={canStudy ? 'bg-purple-600 hover:bg-purple-700' : 'opacity-50'}
-                            onClick={() => store.startStudyingSpell(id)}
+                            onClick={() => store.setCurrentStudy?.(id, 'spell')}
                           >
                             Start Study ({fmt(unlockCost)} mana)
                           </Button>