[codex] Persist custom model selection

[Star-Star66]

Summary

• persist explicit /model selections in ~/.carboncode/config.json
• apply the saved model consistently in CLI, ACP, desktop, QQ, and dashboard entry points
• clear an explicit model pin when the user selects a preset, so presets regain ownership of model selection
• keep startup precedence explicit: --model > saved model > preset default

Why

Selecting a model that is not one of the built-in flash/pro presets only changed the current process. After restarting Carbon Code, the CLI silently returned to the preset model. This was especially confusing for custom or OpenAI-compatible model IDs.

This change preserves the user's explicit model choice across restarts without expanding the scope into full multi-provider configuration. Existing configs remain compatible because the new model field is optional.

Validation

• npm run verify
• 338 test files passed
• 3918 tests passed, 2 skipped

[Cyberforker]

Review Summary

Reviewed & merged by Carbon Code

Tests: npm run verify - 3918/3918 passed, 2 skipped
Feat: persist explicit custom model selections across CLI, ACP, desktop, and dashboard entry points.

Thanks @Star-Star66 for the thorough model-persistence implementation and coverage across entry points.

Co-Authored-By: Carbon Code carboncode@code.ai6666.com

-- Carbon Code