Fix older-than filter to use descendant mtime instead of directory mtime

When checking if a directory should be purged based on --older-than,
the retention filter now uses only the newest descendant file mtime,
ignoring the directory's own metadata timestamp. This prevents directories
with stale contents but recently updated metadata from being incorrectly
skipped during purge operations.

Author: cursoragent

src/utils/purge-storage/planning.ts

@@ -130,8 +130,7 @@ async function candidateFromPath(params: {
   }
 
   const scan = await scanPath(params.candidatePath);
-  const effectiveMtimeMs =
-    scan.latestMtimeMs === null ? stat.mtimeMs : Math.max(stat.mtimeMs, scan.latestMtimeMs);
+  const effectiveMtimeMs = scan.latestMtimeMs ?? stat.mtimeMs;
 
   const retentionReason = shouldKeepForRetention({
     mtimeMs: effectiveMtimeMs,