Add display.theme_mode setting (auto/light/dark) that makes the CLI
readable on light terminal backgrounds.
- Auto-detect terminal background via COLORFGBG, OSC 11, and macOS
appearance (fallback chain in hermes_cli/colors.py)
- Add colors_light overrides to all 7 built-in skins with dark/readable
colors for light backgrounds
- SkinConfig.get_color() now returns light overrides when theme is light
- get_prompt_toolkit_style_overrides() uses light bg colors for
completion menus in light mode
- init_skin_from_config() reads display.theme_mode from config
- 7 new tests covering theme mode resolution, detection fallbacks,
and light-mode skin overrides
Salvaged from PR #1187 by @peteromallet. Core design preserved;
adapted to current main (kept all existing helpers, tool_emojis,
convenience functions that were added after the PR branched).
Co-authored-by: Peter O'Mallet <peteromallet@users.noreply.github.com>
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