fix(audit): clarify missing drift artifacts-dir errors

Replace raw ENOENT from drift artifact directory scans with a clear explicit error message containing the resolved path. Architecture/protocol diagrams reviewed; no updates needed.

docs/plans/state.json

@@ -617,6 +617,17 @@
       ],
       "test_status": "pnpm typecheck + HOME=/tmp path-expansion smoke checks passing"
     },
+    "phase0-live-baseline-drift-missing-dir-error-clarity": {
+      "status": "completed",
+      "date": "2026-02-27",
+      "updated": "2026-02-27",
+      "summary": "Improved drift CLI operator feedback by translating missing artifacts-directory `ENOENT` failures into a clear explicit error message with the resolved path.",
+      "files_modified": [
+        "scripts/check-phase0-baseline-backend-drift.ts",
+        "docs/plans/state.json"
+      ],
+      "test_status": "pnpm typecheck + node --import tsx/esm scripts/check-phase0-baseline-backend-drift.ts --artifacts-dir <missing-dir> --backend native --format json passing (clear error)"
+    },
     "phase0-instrumentation-ticket-checklist": {
       "status": "completed",
       "date": "2026-02-25",

scripts/check-phase0-baseline-backend-drift.ts

@@ -209,7 +209,15 @@ function buildThresholds(values: Record<string, string | boolean | undefined>):
 }
 
 async function readArtifactRecords(artifactsDir: string): Promise<ArtifactRecord[]> {
-  const files = await readdir(artifactsDir);
+  let files: string[];
+  try {
+    files = await readdir(artifactsDir);
+  } catch (error) {
+    if ((error as NodeJS.ErrnoException)?.code === 'ENOENT') {
+      throw new Error(`Artifacts directory does not exist: ${artifactsDir}`);
+    }
+    throw error;
+  }
   const records: ArtifactRecord[] = [];
 
   for (const file of files) {