[AI] Implement provider trait and Anthropic client

[copyleftdev]

Task

Implement AI provider abstraction and Anthropic Claude integration.

Provider Trait

#[async_trait]
pub trait AiProvider: Send + Sync {
    async fn complete(&self, prompt: &str) -> Result<String>;
    fn name(&self) -> &'static str;
}

Anthropic Implementation

• Use reqwest for HTTP
• Read ANTHROPIC_API_KEY from environment
• Use Claude Sonnet model
• System prompt for Genesis file generation

Acceptance Criteria

• Provider trait defined
• Anthropic client implements trait
• Proper error handling for API failures
• Rate limiting awareness
• Timeout handling