feat(provider): expose Atomic Chat in /provider picker with autodetect (#810)

Adds Atomic Chat as a first-class preset inside the in-session /provider
slash command, mirroring the Ollama auto-detect flow. Picking it probes
127.0.0.1:1337/v1/models, lists loaded models for direct selection, and
falls back to "Enter manually" / "Back" when the server is unreachable
or no models are loaded. README updated to reflect the new setup path.

Made-with: Cursor

src/components/ProviderManager.test.tsx

@@ -108,6 +108,7 @@ const PRESET_ORDER = [
   'Alibaba Coding Plan',
   'Alibaba Coding Plan (China)',
   'Anthropic',
+  'Atomic Chat',
   'Azure OpenAI',
   'Codex OAuth',
   'DeepSeek',