Add TOML format support

[krisarmstrong]

Description

Add TOML format support for reading/writing configuration files.

Tasks

• Add TOML reading with tomli (Python <3.11) / tomllib (Python 3.11+)
• Add TOML writing with tomli-w
• Add TOML ↔ JSON conversion
• Add TOML ↔ YAML conversion
• Add tests for conversions
• Update CLI to detect .toml extension
• Update documentation

Implementation Notes

• Use tomli for reading (already added to dependencies)
• Add tomli-w for writing
• TOML is primarily for configs, focus on JSON/YAML interop

Success Criteria

• Can read and write TOML files
• Conversions work correctly
• Tests pass