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/components/game/tabs/index.ts

@@ -5,3 +5,5 @@ export { CraftingTab } from './CraftingTab';
 export { SpireTab } from './SpireTab';
 export { SpellsTab } from './SpellsTab';
 export { LabTab } from './LabTab';
+export { SkillsTab } from './SkillsTab';
+export { StatsTab } from './StatsTab';