v0.3.1

Release v0.3.1

Features:
- Automatic backup prompts when modifying vault files (default: Yes)
- Single backup prompt for GPG encryption (default: Yes)
- ASCII armor prompt now defaults to Yes
- Improved backup file naming (preserves extensions)
- Overwrite protection with user prompts
- Detailed encryption specification in README

Improvements:
- Regular backups: vault.toml.backup.{timestamp}
- GPG backups: vault.toml.gpg.backup.{timestamp}
- Consistent Y/n prompting throughout
- Better error handling and user experience

Documentation:
- Added comprehensive encryption specification
- Updated README with backup functionality
- Updated CLAUDE.md with v0.3.1 changes