Support multiple LLM providers with auto-detection

[ajianaz]

Description

Allow cora-cli to automatically detect which LLM providers are available based on environment variables and provide a fallback chain.

Acceptance Criteria

• Auto-detect provider from env vars: OPENAI_API_KEY → OpenAI, ANTHROPIC_API_KEY → Anthropic, GOOGLE_API_KEY → Google
• Support provider fallback chain in config: providers: [openai, anthropic]
• Auto-select best available model for the provider
• Log which provider/model is being used
• Support custom OpenAI-compatible endpoints (Ollama, vLLM, etc.)

Config Example

provider:
 auto_detect: true
 fallback:
 - name: openai
 api_key_env: OPENAI_API_KEY
 - name: anthropic
 api_key_env: ANTHROPIC_API_KEY

[ajianaz]

Implemented in PR #24, merged to develop.