fix: update marketplace.json to use explicit GitHub source format

Replace relative path source ("./") with the canonical GitHub source
object format ({ "source": "github", "repo": "owner/repo" }).

This resolves schema validation errors seen in Claude CLI v2.1.156+
where the relative path format caused 'Invalid input' errors in
strict marketplace JSON validators.

Also adds:
- $schema reference for editor autocomplete and validation
- top-level description field (was nested under metadata)
- owner URL, homepage, license, and keywords fields

Closes #212

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.claude-plugin/marketplace.json

@@ -1,16 +1,22 @@
 {
+  "$schema": "https://json.schemastore.org/claude-code-marketplace.json",
   "name": "addy-agent-skills",
+  "description": "Production-grade engineering skills for AI coding agents — covering the full software development lifecycle from spec to ship.",
   "owner": {
-    "name": "Addy Osmani"
-  },
-  "metadata": {
-    "description": "Production-grade engineering skills for AI coding agents — covering the full software development lifecycle from spec to ship."
+    "name": "Addy Osmani",
+    "url": "https://github.com/addyosmani"
   },
   "plugins": [
     {
       "name": "agent-skills",
-      "source": "./",
-      "description": "Production-grade engineering skills covering every phase of software development: spec, plan, build, verify, review, and ship."
+      "source": {
+        "source": "github",
+        "repo": "addyosmani/agent-skills"
+      },
+      "description": "Production-grade engineering skills covering every phase of software development: spec, plan, build, verify, review, and ship.",
+      "homepage": "https://github.com/addyosmani/agent-skills",
+      "license": "MIT",
+      "keywords": ["skills", "agents", "engineering", "spec", "tdd", "review", "ship"]
     }
   ]
 }