test: Add Phase 0 safety net tests for refactor

- Add StepExecutor parallel execution tests with race condition check
- Add git.IsMerged error scenario tests
- Add git.ListWorktrees porcelain parsing tests
- Add StepConditionEvaluator tests documenting differences from ConditionEvaluator
- Add config package tests (0% coverage -> ~100%)
- Add CLI input handling tests confirming panic on empty input
- Document current behavior and test gaps in idiomatic-refactor.md

Author: Test User