[Good first issue]: Add a Phoenix recorded-score adapter example

[thangldw]

User outcome

A Phoenix user can export already-recorded evaluator scores into RAGOps without adding Phoenix to the dependency-free core.

Scope

• Add a small example under examples/ or the adapter documentation.
• Map a finite per-case Phoenix annotation score into the existing external-metrics envelope.
• Include a recorded synthetic input, expected output, and a test.
• Document score direction and provenance ownership without recalibration.

Acceptance criteria

• The example imports no Phoenix SDK from src/ragops/.
• Producer score meaning, scale, and direction remain unchanged.
• Invalid or missing case coverage fails closed.
• ruff check . and PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 pytest -q pass.

Non-goals

Do not initialize tracing, call a hosted service, add credentials, or claim Phoenix and RAGOps metrics are equivalent.