feat(core): implement Anthropic API client with streaming

Add anthropic.rs with complete Claude API integration:
- AnthropicClient with builder pattern configuration
- Message types (Role, ContentBlock, Tool, etc.)
- Non-streaming message() method with retry logic
- Streaming message_stream() with SSE parsing
- Comprehensive error types (AuthError, RateLimited, ApiError)
- Exponential backoff for transient errors
- 8 unit tests for client and parsing

Dependencies added: reqwest (with stream), futures

Closes #19

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Cargo.toml

@@ -51,6 +51,9 @@ serde_json = "1"
 tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter"] }
 
+# HTTP Client
+reqwest = { version = "0.12", features = ["json", "stream"] }
+
 # Utilities
 chrono = { version = "0.4", features = ["serde"] }
 uuid = { version = "1", features = ["v4", "serde"] }