docs: Add README.md, update AGENTS.md, audit report, and massive refactoring

Documentation:
- Add comprehensive README.md with project overview
- Update AGENTS.md with new file structure and slice pattern
- Add AUDIT_REPORT.md documenting unimplemented effects

Refactoring (page.tsx: 1695 → 434 lines, 74% reduction):
- Extract SkillsTab.tsx component
- Extract StatsTab.tsx component
- Extract UpgradeDialog.tsx component
- Move getDamageBreakdown and getTotalDPS to computed-stats.ts
- Move ELEMENT_ICON_NAMES to constants.ts

All lint checks pass, functionality preserved.

src/lib/game/store.ts

@@ -72,6 +72,8 @@ import {
   getIncursionStrength,
   canAffordSpellCost,
   deductSpellCost,
+  getTotalDPS,
+  getDamageBreakdown,
 } from './computed-stats';
 
 // Re-export formatting functions and computed stats for backward compatibility
@@ -87,6 +89,9 @@ export {
   getIncursionStrength, 
   canAffordSpellCost,
   getFloorMaxHP,
+  getActiveEquipmentSpells,
+  getTotalDPS,
+  getDamageBreakdown,
 };
 
 // ─── Local Helper Functions ────────────────────────────────────────────────────