Add NEAR AI chat-api as default LLM provider

Adds NearAiProvider that uses the NEAR AI unified API at
api.near.ai/v1/responses with session token authentication.
This provides access to multiple models (OpenAI, Anthropic, etc.)
through a single endpoint with user auth and usage tracking.

- Add src/llm/nearai.rs with complete provider implementation
- Add NearAiConfig to config.rs with session_token, model, base_url
- Add NearAi variant to LlmProvider enum (accepts nearai/near-ai/near_ai)
- Change default provider from OpenAi to NearAi
- Update .env.example with NEAR AI configuration
- Update CLAUDE.md documentation

Co-Authored-By: Claude Opus 4.5 <[email protected]>

.env.example

@@ -3,13 +3,23 @@ DATABASE_URL=postgres://near_agent:password@localhost:5432/near_agent
 DATABASE_POOL_SIZE=10
 
 # LLM Providers
+# Default is NEAR AI which provides a unified interface to all models
+
+# NEAR AI (recommended - unified API with user authentication)
+NEARAI_SESSION_TOKEN=sess_...
+NEARAI_MODEL=claude-3-5-sonnet-20241022
+NEARAI_BASE_URL=https://api.near.ai
+
+# OpenAI (alternative)
 OPENAI_API_KEY=sk-...
 OPENAI_MODEL=gpt-4-turbo-preview
+
+# Anthropic (alternative)
 ANTHROPIC_API_KEY=sk-ant-...
 ANTHROPIC_MODEL=claude-3-opus-20240229
 
-# Default LLM provider: openai or anthropic
-LLM_PROVIDER=openai
+# Default LLM provider: nearai, openai, or anthropic
+LLM_PROVIDER=nearai
 
 # Channel Configuration
 # CLI is always enabled