Flatten PlanetVim menu style choices

[fedorenchik]

Motivation

• Improve discoverability by moving the Emoji/Plain/Descriptive style selectors out of a nested "Menu Style" submenu and making them direct entries under the PlanetVim root while preserving their semantics and ordering.
• Keep existing UI behavior (radio-state indicators and preference persistence) while simplifying the menu layout and documentation.

Description

• Replace the nested PlanetVim -> Menu Style.* submenu entries with flattened entries PlanetVim -> Menu Style: Emoji, PlanetVim -> Menu Style: Plain, and PlanetVim -> Menu Style: Descriptive in autoload/planet/menu/planet.vim and adjust their menu ordering slots.
• Update menu-related tests to assert the new flattened paths, including tests/test_menu_styles.vim (verify the old submenu is absent and the new entries exist) and tests/test_menu_state.vim (adjust expected menu_info() paths for style assertions).
• Revise user-facing documentation in docs/GUIDE.md, the built-in help doc/planetvim.txt, and docs/MENU_REVIEW.md to describe the direct Menu Style: entries and the new wording.

Testing

• Ran python3 -m unittest tests.test_menu_contract and it passed successfully.
• Ran python3 -m unittest discover -s tests -p 'test_*.py' and the suite completed successfully (115 tests run, 14 skipped).
• Attempted to run the Vimscript runtime checks with python3 scripts/test.py tests/test_menu_styles.vim tests/test_menu_state.vim, but those tests could not execute because GVim was not available in the environment.

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Codex Task