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]>

CLAUDE.md

@@ -55,6 +55,7 @@ src/
 │
 ├── llm/                # LLM integration
 │   ├── provider.rs     # LlmProvider trait, message types
+│   ├── nearai.rs       # NEAR AI chat-api (default, unified interface)
 │   ├── openai.rs       # OpenAI API implementation
 │   ├── anthropic.rs    # Anthropic API implementation
 │   └── reasoning.rs    # Planning, tool selection, evaluation
@@ -162,12 +163,37 @@ Pending -> InProgress -> Completed -> Submitted -> Accepted
 Environment variables (see `.env.example`):
 ```bash
 DATABASE_URL=postgres://user:pass@localhost/near_agent
+
+# LLM Provider (default: nearai)
+LLM_PROVIDER=nearai  # Options: nearai, openai, anthropic
+
+# NEAR AI (recommended - unified API with user auth)
+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
+
+# Anthropic (alternative)
 ANTHROPIC_API_KEY=sk-ant-...
+ANTHROPIC_MODEL=claude-3-opus-20240229
+
+# Agent settings
 AGENT_NAME=near-agent
 MAX_PARALLEL_JOBS=5
 ```
 
+### NEAR AI Provider
+
+The default provider uses the NEAR AI chat-api (`https://api.near.ai/v1/responses`) which provides:
+- Unified access to multiple models (OpenAI, Anthropic, etc.)
+- User authentication via session tokens
+- Usage tracking and billing through NEAR AI
+
+Session tokens have the format `sess_xxx` (37 characters). They are authenticated against the NEAR AI auth service.
+
 ## Database
 
 Migrations in `migrations/`. Tables: