- Remove *** placeholder detection from _sanitize_env_lines (was based on
confusing terminal redaction with literal file content)
- Add migrate_config() logic to clear stale ANTHROPIC_TOKEN when better
credentials exist (ANTHROPIC_API_KEY or Claude Code auto-discovery)
- Old ANTHROPIC_TOKEN values shadow Claude Code credential fallthrough,
breaking auth for users who updated without re-running setup
- Preserves ANTHROPIC_TOKEN when it's the only auth method available
- 3 new migration tests, updated existing tests