fix: use object-shaped author field and surface descriptions in marketplace.json

Claude Code's plugin/marketplace schemas require author to be an object
({"name": ...}), not a bare string — installing any plugin from this
marketplace failed validation ("author: Invalid input: expected object,
received string"). Convert every plugins/*/plugin.json author field
accordingly, and add a build-time check so this can't regress.

Also propagate each plugin's description/author into the generated
marketplace.json entries (not just its own nested plugin.json), matching
Anthropic's official marketplace shape, so the Discover UI shows a
description before install instead of a blank "Plugin details" pane.

plugins/core-essentials/plugin.json

@@ -2,5 +2,5 @@
   "name": "core-essentials",
   "description": "Core cross-tool utilities, including delegation to the Antigravity CLI (agy) for grounded research and large multi-file audits.",
   "version": "1.0.0",
-  "author": "Rootiest"
+  "author": { "name": "Rootiest" }
 }