Add showInstallCountsForOfficialOnly flag to marketplace config

Only show install counts in the plugin marketplace for official
claude-plugins-official plugins (those with author.email set to
support@anthropic.com). Unofficial/external plugins will not
display install counts.

Slack thread: https://anthropic.slack.com/archives/C09UPL58BUZ/p1770933725150989

https://claude.ai/code/session_01Ba5UH1rQKgRc3WznA6RAoc

.claude-plugin/marketplace.json

@@ -6,6 +6,7 @@
     "name": "Anthropic",
     "email": "support@anthropic.com"
   },
+  "showInstallCountsForOfficialOnly": true,
   "plugins": [
     {
       "name": "typescript-lsp",