Retire legacy run_llm_assessment fallback path

[simaba]

Problem

src/phases/__init__.py::run_llm_assessment() is a legacy public helper that:

• describes itself as the primary entry point even though the supported path is now src.engine.run_assessment() → run_assessment_with_provenance();
• hard-codes the Anthropic client and claude-sonnet-4-6;
• silently returns _deterministic_fallback(...) when the API key is absent or any exception occurs;
• bypasses the provenance-aware fallback result and certainty-softening logic in src/assessment_service.py.

This means callers that import the legacy helper can receive fallback output without the llm / deterministic_fallback provenance contract and with stronger raw fallback wording than the supported service path.

Preferred fix

Replace the implementation with a deprecated compatibility wrapper:

def run_llm_assessment(project: ProjectInput, mode: str, audience: str) -> AssessmentResult:
    """Deprecated compatibility wrapper for provenance-aware assessment generation."""
    from src.assessment_service import run_assessment_with_provenance

    return run_assessment_with_provenance(project, mode, audience)

The local import avoids the circular import created by assessment_service importing low-level helpers from src.phases.

Alternatively, remove the helper after confirming there are no external compatibility requirements.

Acceptance criteria

• legacy helper no longer owns provider/model selection;
• no silent fallback outside assessment_service;
• supported and compatibility paths return the same provenance-aware result contract;
• tests cover missing API key and provider exception behavior;
• documentation continues to identify src.engine.run_assessment() as the supported application entry point.