refactor(claude): create modular entry point and finalize

Complete the refactoring (Phase 6):

- index.mjs: New modular entry point with CLI routing
- claude.mjs: Updated to delegate to modular implementation
- Removed REFACTORING_PLAN.md (work complete)

The monolithic 5,723-line claude.mjs has been successfully refactored
into 25+ focused modules:

Structure:
- Core: config, formatting, cost/progress tracking, model strategy
- Execution: storage, command execution, authentication, parallelization
- Features: error analysis, project sync, automated fixing
- Commands: 15 self-contained command modules

Benefits:
- Maintainability: No file > 1.5K lines (except green at 1.5K)
- Testability: Modules can be unit tested independently
- Clarity: Single responsibility per module
- Extensibility: Easy to add new commands

All modules follow CLAUDE.md standards with proper imports, sorting,
and the __proto__: null pattern.

Author: jdalton