fix(audit): tolerate missing phase0 artifact dir in prune

Treat missing artifacts dir as empty input for prune planning/apply workflows on fresh hosts; keep report output behavior intact. Architecture/protocol diagrams reviewed; no updates needed for this script robustness change.

scripts/prune-phase0-baseline-artifacts.ts

@@ -15,7 +15,7 @@ function usage(): string {
     '',
     'Options:',
     '  --artifacts-dir <path>     Artifacts directory (default: docs/plans/artifacts)',
-    '  --keep-per-family <num>    Keep newest rolling tags per family (default: 8)',
+    '  --keep-per-family <integer> Keep newest rolling tags per family (default: 8)',
     '  --apply                    Apply deletions (default: dry-run)',
     '  --report-tag <tag>         Report tag suffix (default: current UTC date)',
     '  --write-default-artifacts  Write report files to artifacts dir',
@@ -78,6 +78,17 @@ async function writeTextFile(pathValue: string, contents: string): Promise<void>
   await writeFile(pathValue, `${contents}\n`, 'utf8');
 }
 
+async function readArtifactFileNames(artifactsDir: string): Promise<string[]> {
+  try {
+    return await readdir(artifactsDir);
+  } catch (error) {
+    if ((error as NodeJS.ErrnoException)?.code === 'ENOENT') {
+      return [];
+    }
+    throw error;
+  }
+}
+
 async function main(): Promise<void> {
   const { values } = parseArgs({
     options: {
@@ -123,7 +134,7 @@ async function main(): Promise<void> {
       ? `${defaultBaseName}.md`
       : undefined;
 
-  const files = await readdir(artifactsDir);
+  const files = await readArtifactFileNames(artifactsDir);
   const plan = planRollingPhase0ArtifactRetention(files, keepPerFamily);
 
   if (apply) {