Reorganize Extension project structure

[KubrickCode]

Goal: Improve directory structure following project-structure skill principles

Task Checklist:

• src/extension/extension/main.ts: Move entry point
• src/extension/internal/managers/: Move Manager classes
• src/extension/internal/providers/: Move Provider classes
• src/extension/pkg/types.ts: Move public types
• src/extension/tests/: Move test files and rename to .spec.ts
• Update import paths
• Update package.json main field
• Build verification

Verification Method:

1. Build succeeds with new directory structure
2. Extension loads and activates normally
3. All commands execute correctly

Independently Deployable: ✅ (File relocation only, no functional changes)