fix: address PR Agent review findings across 6 files

Restrict PR Agent workflow to trusted collaborators on PR comments only,
fix UTF-8 byte counting in gallery-pack, correct portrait-to-landscape
fallback orientation, track inserted freeform beats in visitedBeatIds,
allow clearing stored TTS key, and guard empty-string fuzzy match in
style selector.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/engine/agents/styleSelector.ts

@@ -28,7 +28,9 @@ export async function selectStyle(
   if (STYLE_MAP[picked]) {
     return STYLE_MAP[picked];
   }
-  const fuzzy = STYLE_NAMES.find((s) => picked.includes(s) || s.includes(picked));
+  const fuzzy = picked
+    ? STYLE_NAMES.find((s) => picked.includes(s) || s.includes(picked))
+    : undefined;
   if (fuzzy) {
     return STYLE_MAP[fuzzy]!;
   }