chore(catalog): remove plugins that are no longer published

Remove claude-design from the marketplace manifest and its catalog
README entry, and remove the playground-design-system entry from the
README. The catalog now lists 11 plugins.

Add scripts/check-public-links.mjs: fails when README.md or the
manifest links to an open/<name> repository missing from a tracked
PUBLIC_REPOS list. Chosen over the local retired-terms list because
that gate scans every tracked file, and historical docs plus
CONVENTIONS.md (which keep naming these repos on purpose) would hold
it red; a live visibility check could not be red before the repos
change visibility. Red on 5ba4a7a (6 hits), green after (0 of 36).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.claude-plugin/marketplace.json

@@ -91,15 +91,6 @@
       },
       "description": "Shared Claude Code output style for the ktg-plugin-marketplace. Plain-language tone — explains what and why, hides paths/JSON/stack traces by default, matches the user's language."
     },
-    {
-      "name": "claude-design",
-      "source": {
-        "source": "url",
-        "url": "https://git.fromaitochitta.com/open/claude-design.git",
-        "ref": "v0.2.0"
-      },
-      "description": "End-to-end facilitator for prompting Claude Design (claude.ai/design) — idea to copy-paste-ready prompt with iteration coaching, citing Anthropic primary sources."
-    },
     {
       "name": "repo-mailbox",
       "source": {