Re-enable hidden model providers (local + cloud-API) — temporarily limited to subscription CLIs

[100rd]

Context

The UI and chat model list are temporarily limited to the subscription-CLI providers — Antigravity (antigravity, mirrored onto google) and Claude (anthropic CLI). All other providers do not work in the current setup and were hidden (not removed) to avoid offering non-functional options.

Hidden providers

• Local models: vLLM, Ollama, LM Studio
• Cloud APIs: xAI (Grok), the billed Gemini API, and the Anthropic/Google API-key paths
• The Settings sections that configure them: Local models, Cloud Providers (API keys), Discover Models, Probe Endpoint, Add Model, Registered Models

How it's hidden (reversible by design — no code removed)

• Server: server/gateway/index.ts → VISIBLE_PROVIDER_KEYS allowlist gates discoverModels() (chat model list) and getStatus(). Provider registration is left intact.
• Client: client/src/pages/Settings.tsx → the provider/model-config cluster is gated behind PROVIDER_CONFIG_UI_ENABLED (false) with an explanatory banner.

Re-enable (follow-up PR)

1. Widen VISIBLE_PROVIDER_KEYS to include the keys to expose (e.g. vllm, ollama, lmstudio, xai).
2. Flip PROVIDER_CONFIG_UI_ENABLED to true (or make it config-driven).
3. Verify the provider actually works (endpoint/API key reachable) before exposing it.

Tracking issue for the PR that introduced the hide.