Implement PrometheusAdapter.fetch_metric()

[anulum]

Summary

PrometheusAdapter.fetch_metric() in src/scpn_phase_orchestrator/adapters/prometheus.py raises NotImplementedError.

Acceptance Criteria

• Query Prometheus HTTP API (/api/v1/query_range)
• Return NDArray of metric values over [start, end] at step resolution
• Handle connection errors, empty results, and malformed responses
• Unit tests with mocked HTTP responses

Context

Originally scoped for v0.3 per ROADMAP.md. Deferred.