[codex] Add multi-provider chat switching

[Star-Star66]

Summary

• add named OpenAI-compatible chat providers while preserving legacy apiKey / baseUrl configuration
• add /provider [name] to list and switch providers without restarting the active TUI session
• reload provider credentials, endpoint, and optional default model through the existing runtime config watcher
• document the command and cover configuration, runtime reload, slash help, and suggestion behavior

Why

Issue #95 item 3 asks for multiple provider configuration and in-conversation model/provider switching. Carbon Code previously stored only one chat connection, so changing providers required overwriting the current credentials and endpoint.

Compatibility

Existing single-provider DeepSeek configurations continue to resolve as the implicit deepseek provider. DEEPSEEK_API_KEY and DEEPSEEK_BASE_URL remain supported as explicit environment overrides.

Validation

• npm run verify
• 348 test files passed
• 3996 tests passed, 2 skipped

Addresses #95 item 3.

[Cyberforker]

👋 Thanks for the PR @Star-Star66! This is marked as draft — I'll review it when it's ready for merge. No action needed now.

— Carbon Code

[Cyberforker]

Review Summary

Draft PR reviewed — no action needed yet.

This PR adds multi-provider chat switching with /provider slash command, ChatProviderConfig, and runtime config reload. The scope and compatibility story look solid. Will perform a full review and merge when the draft is marked ready.

Thanks @Star-Star66 for tackling the multi-provider feature!

-- Carbon Code