feat: Add multi-provider LLM support via rig-core adapter (#36)

Add support for OpenAI, Anthropic, Ollama, and OpenAI-compatible
endpoints alongside the existing NEAR AI backend. Users can now
bring their own API keys via environment variables (LLM_BACKEND,
OPENAI_API_KEY, ANTHROPIC_API_KEY, etc.) while NEAR AI remains
the default.

Co-authored-by: Claude Opus 4.6 <[email protected]>

src/setup/wizard.rs

@@ -455,6 +455,7 @@ impl SetupWizard {
             .unwrap_or_else(|_| "https://private.near.ai".to_string());
 
         let config = LlmConfig {
+            backend: crate::config::LlmBackend::NearAi,
             nearai: crate::config::NearAiConfig {
                 model: "dummy".to_string(),
                 base_url,
@@ -463,6 +464,10 @@ impl SetupWizard {
                 api_mode: crate::config::NearAiApiMode::Responses,
                 api_key: None,
             },
+            openai: None,
+            anthropic: None,
+            ollama: None,
+            openai_compatible: None,
         };
 
         match create_llm_provider(&config, Arc::clone(session)) {