[UX/Docs] Resolve naming debt: VIBEMODE_API_KEY vs NEUROGATE_API_KEY

[xodapi]

Currently, our documentation (SECURITY.md, README.ru.md), codebase, and error messages have a mix of the old NEUROGATE_API_KEY branding and the new VIBEMODE_API_KEY. This is causing confusion during the first setup.

Tasks:

• Search and replace NEUROGATE_API_KEY with VIBEMODE_API_KEY in README.ru.md and SECURITY.md.
• Update any error hints in the CLI (e.g., in --init or --doctor) to reference the new key name.
• Add a migration fallback in code (if VIBEMODE_API_KEY is not found, check NEUROGATE_API_KEY and print a warning to update it).

[xodapi]

🤖 Начинаю выполнение. Агент: Factory Droid

[xodapi]

✅ PR готов: #41

[xodapi]

✅ Выполнено в PR #41 (merged)