Feature: GitWorktreeSandbox class methods for stale cleanup and diff
  As a developer
  I want class-level helpers to clean up stale worktrees and generate diffs
  So that the CLI can manage sandbox lifecycle without a sandbox instance

  Background:
    Given a gwt_cm test git repository is initialised

  Scenario: cleanup_stale removes a stale worktree branch
    Given a gwt_cm stale worktree branch exists for plan "plan-stale-001"
    When I call GitWorktreeSandbox.cleanup_stale for plan "plan-stale-001"
    Then the gwt_cm stale branch should no longer exist

  Scenario: cleanup_stale is idempotent when no stale branch exists
    When I call GitWorktreeSandbox.cleanup_stale for plan "plan-nonexistent-999"
    Then no gwt_cm exception should have been raised

  Scenario: cleanup_stale handles empty plan_id gracefully
    When I call GitWorktreeSandbox.cleanup_stale with empty plan_id
    Then no gwt_cm exception should have been raised

  Scenario: cleanup_stale handles empty original_path gracefully
    When I call GitWorktreeSandbox.cleanup_stale with empty original_path
    Then no gwt_cm exception should have been raised

  Scenario: diff_against_head returns None when no worktree branch exists
    When I call GitWorktreeSandbox.diff_against_head for plan "plan-no-branch-001"
    Then the gwt_cm diff result should be None

  Scenario: diff_against_head returns diff when worktree branch has changes
    Given a gwt_cm worktree branch with changes exists for plan "plan-diff-001"
    When I call GitWorktreeSandbox.diff_against_head for plan "plan-diff-001"
    Then the gwt_cm diff result should not be None

  Scenario: diff_against_head handles empty plan_id gracefully
    When I call GitWorktreeSandbox.diff_against_head with empty plan_id
    Then the gwt_cm diff result should be None

  Scenario: diff_against_head handles empty original_path gracefully
    When I call GitWorktreeSandbox.diff_against_head with empty original_path
    Then the gwt_cm diff result should be None