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/mattpocock-skills/plugin.json

@@ -2,7 +2,7 @@
   "name": "mattpocock-skills",
   "description": "Matt Pocock's agent skills for real engineering from mattpocock/skills — grilling, spec/ticket flows, TDD, code review, domain modelling and more. Installed straight from the upstream repo, not vendored here.",
   "version": "1.0.0",
-  "author": "Rootiest",
+  "author": { "name": "Rootiest" },
   "targets": ["claude-code"],
   "external": {
     "claude-code": {