refactor: resolve structural inconsistencies and dead code

- Fix broken barrel exports in components/game/index.ts
- Remove skill system from stores (gameStore, gameActions, gameLoopActions, gameHooks, craftingStore, combat)
- Remove skill system from components (page.tsx, LeftPanel, StatsTab, SpellsTab, EnchantmentDesigner, EnchantmentPreparer, GameContext/Provider)
- Delete dead code: stats/ directory, attunements/ directory, layout/ Header+TabBar, shared/ StudyProgress+UpgradeDialog duplicates, effects.ts.fix, study-slice.ts, navigation-slice.ts
- Delete legacy store/ and store-modules/ directories, redirect remaining callers
- Merge root formatting.ts into utils/formatting.ts
- Move effects files (dynamic-compute, upgrade-effects, special-effects, upgrade-effects.types) into effects/ directory
- Move debug-context.tsx into components/game/debug/
- Create tabs/index.ts barrel for tab components
- Fix page.tsx lazy imports to use tabs barrel
- Fix all broken import paths across codebase
- Remove SKILLS_DEF and skill-evolution references
- Trim store.ts to under 400 lines by removing dead skill actions

src/lib/game/constants/index.ts

@@ -16,14 +16,6 @@ export { GUARDIANS } from './guardians';
 // Spell constants
 export { SPELLS_DEF } from './spells';
 
-// Skill constants (legacy)
-export { SKILLS_DEF, SKILL_CATEGORIES, EFFECT_RESEARCH_MAPPING, BASE_UNLOCKED_EFFECTS, ENCHANTING_UNLOCK_EFFECTS } from './skills';
-
-// Skill System v2
-export { computeStats, BASE_STATS, SKILLS_V2 } from './skills-v2';
-export type { ComputedStats, SkillV2Def, SkillEffect, StatKey } from './skills-v2-types';
-export { getBaseSkillId, hasPrerequisites } from './skills-v2';
-
 // Prestige constants
 export { PRESTIGE_DEF } from './prestige';