slice 1.0.1: AnthropicProvider — minimal streaming + tool use

[sausi-7]

Phase 1.0 demo slice · Difficulty: D2

Scope

The smallest workable LLMProvider ABC and a single AnthropicProvider implementation. No factory, no provider switching. Key read from ANTHROPIC_API_KEY env var (BYOK UI is deferred to full Phase 1).

Acceptance criteria

• LLMProvider.stream(messages, tools) -> AsyncIterator[Event] defined.
• Events: token, tool_call, tool_result, done, error.
• AnthropicProvider streams tokens and surfaces tool-use blocks.
• Smoke test (manual is fine for the slice; recorded fixture preferred).

Carries forward into full Phase 1 issue 1.1.2 with no rewrite.