fix(coverage): use sys.executable for the pytest --cov subprocess (ORCH-027)

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>

src/coverage_gate.py

@@ -53,6 +53,7 @@ import json
 import logging
 import os
 import subprocess
+import sys
 
 from .config import settings
 from .git_worktree import ensure_worktree, get_worktree_path
@@ -140,8 +141,11 @@ def measure_coverage(repo: str, branch: str) -> float | None:
     except OSError:
         pass
 
+    # Use the SAME interpreter that runs the orchestrator (sys.executable), not a
+    # bare "python" — the prod container / CI runner expose "python3", and the
+    # pytest-cov plugin lives in exactly this interpreter's environment.
     cmd = [
-        "python", "-m", "pytest", "tests/",
+        sys.executable, "-m", "pytest", "tests/",
         "--cov=src",
         f"--cov-report=json:{cov_json}",
         "--cov-report=",  # suppress the terminal cov report (json only)