feat(cli): make terminal theme detection skin-aware and readable in light mode

[peteromallet]

What does this PR do?

When I opened Hermes for the first time in a light terminal, a bunch of the CLI was still too light to read cleanly — not just the prompt/input area, but also banner/system text and other UI surfaces.

This makes the CLI theme handling more cohesive end-to-end:

• detect whether the terminal is light or dark
• choose defaults that stay readable across the whole interface
• keep the look aligned with the existing skin/design system instead of just forcing everything to black
• still let users override the result if they want

Related Issue

Fixes #

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 🔒 Security fix
• 📝 Documentation update
• ✅ Tests (adding or improving test coverage)
• ♻️ Refactor (no behavior change)
• 🎯 New skill (bundled or hub)

Changes Made

• extend the existing skin system instead of bypassing it: theme handling now sits on top of skins rather than introducing a separate one-off light-mode path
• add display.theme_mode with auto | light | dark
• auto-detect terminal background using OSC 11, with COLORFGBG and macOS appearance fallbacks when needed
• make theme mode fully skin-aware so colors come from the active skin instead of scattered hardcoded values
• add/darken colors_light values across built-in skins so light-mode contrast stays readable without losing the Hermes look
• update banner/system text rendering so the large logo, caduceus, status line, session labels, and related UI all follow the active skin/theme
• stop hardcoding pale input/body text so prompt_toolkit can use terminal defaults where that is safer
• make completion menu and other prompt_toolkit surfaces theme-aware in light/dark mode
• move config writes to the canonical config helper so skin/theme settings are saved in one consistent place
• update setup flow and cli-config.yaml.example to document the new display setting and light-mode overrides
• add tests covering theme mode resolution, auto-detection fallbacks, and light-mode skin overrides

How to Test

1. Start Hermes in a light terminal background and confirm the prompt/input text, banner text, session/status text, and menu surfaces are all readable on first launch.
2. Run /skin theme auto, /skin theme light, and /skin theme dark and confirm the selected mode is reflected correctly.
3. Switch between built-in skins and confirm the colors remain readable in both light and dark terminals.
4. Confirm user overrides still win by setting display.theme_mode in config.

Screenshots / Logs

Before / after on Desktop:

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run pytest tests/ -q and all tests pass
• I've added tests for my changes (required for bug fixes, strongly encouraged for features)
• I've tested on my platform: macOS with a light terminal theme

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — or N/A
• I've updated cli-config.yaml.example if I added/changed config keys — or N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — or N/A
• I've considered cross-platform impact (Windows, macOS) per the compatibility guide — or N/A
• I've updated tool descriptions/schemas if I changed tool behavior — or N/A

Targeted tests related to this change passed:

• python3 -m pytest tests/hermes_cli/test_skin_engine.py tests/hermes_cli/test_config.py tests/hermes_cli/test_setup.py tests/hermes_cli/test_set_config_value.py tests/test_cli_init.py tests/test_cli_loading_indicator.py tests/test_cli_model_command.py tests/test_cli_secret_capture.py -q
• result: 103 passed

I also ran the full suite with python3 -m pytest tests/ -q.
It still does not go fully green in this local environment because 3 existing failures remain outside this change set:

• tests/test_cli_provider_resolution.py::test_codex_provider_uses_config_model
• tests/test_quick_commands.py::TestCLIQuickCommands::test_exec_command_runs_and_prints_output
• tests/test_quick_commands.py::TestCLIQuickCommands::test_quick_command_takes_priority_over_skill_commands

I verified those same 3 failures reproduce on a clean origin/main worktree as well, and this branch does not touch the failing code paths, so they appear to be pre-existing and unrelated to this PR.

Let me know if you have any issues with the approach or want me to tighten up any of the theme defaults.

[teknium1]

Salvaged onto current main — the core design is preserved, adapted to the current codebase (600 commits of drift). Your commit is co-authored.

What landed:

• display.theme_mode setting (auto/light/dark) in config
• Auto-detection via COLORFGBG → OSC 11 → macOS appearance fallback chain
• colors_light overrides on all 7 built-in skins
• SkinConfig.get_color() returns light overrides when theme is light
• get_prompt_toolkit_style_overrides() uses light bg colors for completion menus
• 7 new tests

Adapted for current main: kept all convenience helpers (get_active_prompt_symbol, get_active_goodbye, etc.), tool_emojis, and get_prompt_toolkit_style_overrides() that were added after your branch point. Thanks @peteromallet — great feature.