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.

st-export.py

@@ -92,6 +92,18 @@ def export_cards() -> int:
             json.dump(card, fh, indent=2, ensure_ascii=False, sort_keys=True)
             fh.write("\n")
         n += 1
+
+    # ⚠ PRUNE EXPORTS WHOSE SOURCE IS GONE. Without this, deleting a character in
+    # the UI left `_text/cards/<Name>.json` behind describing a card that no
+    # longer exists — and it reads as current, which is worse than absent. Found
+    # by testing a deletion rather than by reading the code: the write path was
+    # obviously correct and the delete path was simply never written.
+    live = {f[:-4] + ".json" for f in os.listdir(src)
+            if f.lower().endswith(".png") and not f.startswith("default_")} if os.path.isdir(src) else set()
+    for stale in sorted(set(os.listdir(dst)) - live):
+        if stale.endswith(".json"):
+            os.remove(os.path.join(dst, stale))
+            print(f"  pruned _text/cards/{stale} — its PNG is gone")
     return n