feat(providers): add Anthropic Claude provider (`internal/providers/anthropic`)

[Nithwin]

Goal

Add native support for Anthropic models (claude-3-5-sonnet, claude-3-5-haiku).

Requirements

1. Create internal/providers/anthropic/provider.go implementing ai.Provider.
2. Translate Go tool definitions into Anthropic's tool use (input_schema) specification.
3. Register via ai.Register("anthropic", New) inside init().