v2.1.0: refactor: improve code quality and pin dependencies by SHA

- Replace fmt.Sprintf path joins with filepath.Join for cross-platform safety
- Move regexp.MustCompile calls to package-level vars to avoid recompilation
- Add safe type assertions (comma-ok pattern) in ParseChall to prevent panics
- Extract ANSI color codes into constants for readability
- Extract matchFlag function to reduce nesting in UnitTest
- Return error from GetConfig instead of silently swallowing failures
- Pin Docker image and GitHub Actions by SHA digest
- Fix typo "Leading" → "Loading" and modernize interface{} → any

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>