Fix code review findings and add example config
- Extract consolidated keychain module (load + store)
- Use Cliffy EnumType for --type flag instead of unsafe cast
- Set restrictive file permissions on config (0o700 dir, 0o600 file)
- Add Array.isArray guard in config validation
- Add bucket/region validation to init prompts
- Extract s3ConnectionFromConfig helper to reduce duplication
- Remove dead configDir export and redundant error check
- Add config.example.json and gitignore ~/.attic/