refactor: break down large functions into smaller, focused ones

Major refactoring to improve code maintainability:

1. Split main.go (481 lines → 72 lines) into internal/cli package:
   - flags.go: Command-line flag definitions and parsing
   - handlers.go: General command handlers (version, config, list operations)
   - sync_handlers.go: Sync-specific handlers for all sync operations

2. Refactored sync.go to extract logic into separate files:
   - git_operations.go: Git command helpers (merge, push, fetch, etc.)
   - repository_setup.go: Repository initialization and remote configuration
   - branch_sync.go: Branch synchronization helpers

3. Reduced function sizes to meet 30-line guideline:
   - syncBranch: 104 lines → 26 lines
   - SyncRepository: 97 lines → 44 lines
   - main(): 465 lines → 63 lines
   - getAllBranches: 32 lines → 9 lines

All functionality remains the same, but the code is now more modular,
testable, and easier to understand. Each function has a single, clear
responsibility.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

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