Prune card exports whose PNG is gone

Deleting a character in the UI left _text/cards/<Name>.json behind, describing a
card that no longer exists. A stale derived file is worse than a missing one
because it reads as current -- which is the exact reason the pre-commit hook
regenerates _text/ at all, so the script was failing its own premise.

Found by testing a deletion rather than reading the code. The write path was
obviously correct; the delete path had simply never been written.

_text/cards/Aino Vihtanen.json

@@ -1,6 +1,6 @@
 {
-  "avatar": "none",
-  "chat": "Aino Vihtanen - 2026-08-12@16h09m13s142ms",
+  "avatar": "Aino Vihtanen.png",
+  "chat": "Aino Vihtanen - 2026-08-13@18h01m20s857ms",
   "create_date": "2026-08-12T16:09:13.167Z",
   "creatorcomment": "",
   "data": {