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/agent/agent_loop.rs

@@ -1049,7 +1049,7 @@ impl Agent {
             iteration += 1;
             if iteration > MAX_TOOL_ITERATIONS {
                 return Err(crate::error::LlmError::InvalidResponse {
-                    provider: "nearai".to_string(),
+                    provider: "agent".to_string(),
                     reason: format!("Exceeded maximum tool iterations ({})", MAX_TOOL_ITERATIONS),
                 }
                 .into());