Redesign skill system with upgrade trees and tier progression

Major changes:
- Created docs/skills.md with comprehensive skill system documentation
- Rewrote skill-evolution.ts with new upgrade tree structure:
  - Upgrades organized in branching paths with prerequisites
  - Each choice can lead to upgraded versions at future milestones
  - Support for upgrade children and requirement chains
- Added getBaseSkillId and generateTierSkillDef helper functions
- Fixed getFloorElement to use FLOOR_ELEM_CYCLE.length
- Updated test files to match current skill definitions
- Removed tests for non-existent skills

Skill system now supports:
- Levels 1-10 for most skills, level 5 caps for specialized, level 1 for research
- Tier up system: Tier N Level 1 = Tier N-1 Level 10 in power
- Milestone upgrades at levels 5 and 10 with branching upgrade trees
- Attunement requirements for skill access and tier up
- Study costs and time for leveling

src/lib/game/store/index.ts

@@ -0,0 +1,9 @@
+// ─── Store Module Exports ─────────────────────────────────────────────────────
+// Re-exports from main store and adds new computed utilities
+// This allows gradual migration while keeping existing functionality
+
+// Re-export everything from the main store
+export * from '../store';
+
+// Export new computed utilities
+export * from './computed';