feat(tui): base16 theme

[snelling-a]

Add Base16 Default Theme (Dark & Light)

Summary

This PR introduces a new theme, base16-default, for the opencode TUI, following the Base16 philosophy and guidelines.
It provides both dark and light variants using the official base16 default-dark and base16 default-light palettes.

Details

• Color palette: Uses the official base16 default palette for both dark and light modes.
• Philosophy: All color slots are mapped according to the base16 usage guidelines (backgrounds, foregrounds, semantic, and syntax colors).
• No hardcoded hex values: All theme slots reference only the color keys from the palette, ensuring maintainability and consistency.
• Light mode: Inverts base00–base07 for light mode, to reflect the default dark/light palettes.
• Semantic and syntax slots: Appropriately mapped to base08–base0F for variables, errors, warnings, strings, functions, etc.

References

• Base16 Styling Guidelines
• Base16 Default Dark YAML
• Base16 Default Light YAML

Motivation

• Provides a canonical, spec-compliant base16 theme for opencode users.
• Ensures color consistency and accessibility across both dark and light modes.
• Makes it easy for users to build or extend new themes based on the base16 standard.