Add default provider configuration for agent CLI

Summary

Allow configuring the default provider instead of hardcoding auto-detection order.

Current Behavior

Provider is auto-detected in order: anthropic → openrouter → ollama

Desired Behavior

Config file or env var to set default:

# Option 1: env var
export AGENT_DEFAULT_PROVIDER=claude-code

# Option 2: config file ~/.config/agent/config.yaml
default_provider: claude-code
default_model: claude-sonnet-4

Why

claude-code provider (OAuth) is best and cheapest for users with Claude Max/Pro subscription.