Implements runClaudeTurn() — spawns the claude CLI for a single companion
turn using subscription/OAuth auth (strips ANTHROPIC_API_KEY +
ANTHROPIC_AUTH_TOKEN from child env), streaming normalised events (delta,
tool, tool_result, result, error) via onEvent callback.
Includes hermetic test + fake-claude.js fixture that mimics real 2.1.159
stream-json output; zero network/CLI calls in the test suite.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
