measure_coverage hardcoded "python" for the coverage subprocess; the prod container
and the CI runner expose "python3" (a bare "python" may be absent), and pytest-cov
lives in exactly the running interpreter's environment. Use sys.executable so the
measurement always runs under the same interpreter as the orchestrator.
Refs: ORCH-027
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>