fix(evidence): stabilize real Pi authoring

harness/tht/cli/evidence_cmd.py

@@ -117,9 +117,26 @@ def prepare_cmd(
     skill_path = Path(__file__).resolve().parents[2] / ".pi" / "skills" / "tht-evidence-authoring" / "SKILL.md"
     restructurer = PiEvidenceRestructurer(os.environ.get("THT_PI_EXECUTABLE", "pi"), skill_path)
     try:
-        report = prepare_workspace_evidence(root, restructurer=restructurer, upgrade=upgrade)
+        try:
+            max_workers = int(os.environ.get("THT_EVIDENCE_AUTHORING_WORKERS", "1"))
+        except ValueError as error:
+            raise EvidencePreparationError("authoring_workers_invalid") from error
+        report = prepare_workspace_evidence(
+            root,
+            restructurer=restructurer,
+            upgrade=upgrade,
+            max_workers=max_workers,
+        )
     except EvidencePreparationError as error:
-        _emit({"schemaVersion": 1, "operation": "evidence_prepare", "status": "failed", "code": error.code}, json_output)
+        payload = {
+            "schemaVersion": 1,
+            "operation": "evidence_prepare",
+            "status": "failed",
+            "code": error.code,
+        }
+        if error.source_file is not None:
+            payload["sourceFile"] = error.source_file
+        _emit(payload, json_output)
         raise typer.Exit(code=1) from error
     _emit(_preparation_payload(report), json_output)
     if report.findings: