Add comprehensive OAuth authentication support for OpenAI Codex models,
enabling users with ChatGPT Pro/Plus subscriptions to access Codex models
(gpt-5.1-codex, gpt-5.1-codex-mini, gpt-5.2-codex) without API keys.

Implementation includes:

- New OAuth authentication module (src/cli/auth/openai-oauth.ts):
  * PKCE flow with SHA-256 for secure authentication
  * Token management with automatic refresh
  * Support for ChatGPT Pro/Plus subscription access
  * Compatible with OpenAI's OAuth 2.0 endpoint

- Config manager updates (src/cli/core/config-manager.ts):
  * Added openaiOAuth configuration schema with encryption
  * Token storage methods: get, set, clear, has
  * Temporary verifier/state management for OAuth flow
  * Secure token encryption using KeyEncryption

- Modern AI provider integration (src/cli/ai/modern-ai-provider.ts):
  * Automatic OAuth detection for Codex models
  * Custom fetch with bearer token authentication
  * Fallback to API key for non-Codex models
  * Token auto-refresh on expiration

- CLI commands (src/cli/nik-cli.ts):
  * /auth openai - Initialize OAuth flow with authorization URL
  * /auth openai-login - Complete OAuth flow with authorization code
  * /auth openai status - Check OAuth connection status
  * /auth openai logout - Disconnect OAuth session
  * Interactive prompts for code input

Inspired by opencode implementation (anomalyco/opencode#7537)