fix: ploy category → canonical grid name (#578)

The site maps partners.json category directly to the sponsorship-page
category grid (CATEGORIES[].name in partner-program.ts), where the
canonical slot is "Website & landing page builders" — "Website" would
never match. Display surfaces are unaffected (they use categoryShort;
sync confirms README/REGISTRY unchanged).


Claude-Session: https://claude.ai/code/session_01VTdwoFxvAL9yNhkuH1EdpS

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

partners.json

@@ -18,7 +18,7 @@
       "id": "ploy",
       "name": "Ploy",
       "url": "https://ploy.ai",
-      "category": "Website",
+      "category": "Website & landing page builders",
       "categoryShort": "AI website & growth platform",
       "tier": "skill-partner",
       "tagline": "Turns your website into a growth engine",