[Refactoring] Break up WorktreeCreator god class

[renekris]

Problem Statement

WorktreeCreator violates Single Responsibility Principle with 9 private methods handling URL parsing, git operations, external tools, file I/O, and user interaction. 490 lines makes it difficult to test, maintain, and extend.

Current Responsibilities

• URL parsing and validation (_parse_github_url)
• Git remote detection (_detect_git_remote)
• Branch name generation (_generate_branch_name, _sanitize_title)
• Branch conflict handling (_check_branch_conflict)
• Worktree creation (_create_worktree)
• File writing (_write_task_file)
• Editor opening (_open_shuvcode)
• Orchestration (create_from_github_url)

Proposed Solution

Split into focused classes with single responsibilities using dependency injection.

Implementation Steps:

• Create url_parser.py with URL parsing logic
• Create branch_namer.py with branch name generation
• Create worktree_manager.py with worktree management interface
• Create workmux_manager.py implementing worktree manager
• Refactor WorktreeCreator to pure orchestrator (~150 lines)
• Implement dependency injection for all services
• Add unit tests for each new class
• Update integration tests for refactored structure

Alternative Approaches

• Use dependency injection container (e.g., dependency-injector)
• Implement builder pattern for complex initialization
• Use composition over inheritance for flexibility

Area

• Refactoring

Estimated Effort

• Medium (1-2d)

Acceptance Criteria

• WorktreeCreator reduced to < 200 lines (from 490)
• Each new class has single responsibility (< 150 lines)
• All existing tests pass after refactoring
• New classes have proper unit tests
• Dependency injection allows easy testing
• Interfaces/protocols enable alternative implementations

Related Issues

None