fix(AC-332): fall back to ANTHROPIC_API_KEY in build_client_from_settings

[jayscambler]

Summary

build_client_from_settings() didn't fall back to ANTHROPIC_API_KEY env var, unlike the provider registry and judge provider which both do.

Fix

# Before:
if not settings.anthropic_api_key:
    raise ValueError("AUTOCONTEXT_ANTHROPIC_API_KEY is required...")

# After:
api_key = settings.anthropic_api_key or os.getenv("ANTHROPIC_API_KEY", "")
if not api_key:
    raise ValueError("AUTOCONTEXT_ANTHROPIC_API_KEY or ANTHROPIC_API_KEY is required...")

Test plan

• 4 tests: AUTOCONTEXT key, ANTHROPIC_API_KEY fallback, no key raises, deterministic OK
• ruff clean, full suite 4393 passed

[linear]

AC-332 Bug: llm_client.build_client_from_settings doesn't fall back to ANTHROPIC_API_KEY