address claude review: getErrorStatus in feed 429 check, fix reportSubject docs

- PostFeedErrorMessage read error?.status directly; only
  XrpcResponseError carries .status now, so route through
  getErrorStatus() like the rest of detectKnownError (also drops an
  oxlint no-unsafe-member-access suppression).
- reportSubject.ts docblocks contradicted each other about cid
  branding; l.CidString is a plain (unbranded) string alias, so the
  file docstring now says so and the strongRef comments agree.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

oxlint-suppressions.json

@@ -1683,7 +1683,7 @@
       "count": 8
     },
     "typescript/no-unsafe-member-access": {
-      "count": 9
+      "count": 8
     }
   },
   "src/view/com/util/EmptyState.tsx": {