Implement ClaudeProvider for Anthropic API

[bug-ops]

Description

Second LlmProvider implementation targeting Claude API.

Parent: #4

Branch

feat/m3/claude-backend

Files

• crates/zeph-llm/src/claude.rs — ClaudeProvider struct + impl
• crates/zeph-llm/src/lib.rs — add module
• crates/zeph-llm/Cargo.toml — add reqwest (or anthropic crate) dep

Acceptance Criteria

• ClaudeProvider::new(api_key, model) constructor
• chat() calls Claude Messages API
• API key from constructor (passed from config/env)
• Basic retry on rate limit (429) with exponential backoff
• Integration test marked #[ignore] (requires API key)

Depends On

#8 (LlmProvider trait)