fix: use ./ source and simplify plugin.json skills path

Codex review found two issues with the initial fix:
- GitHub source object causes redundant clone, may fail without SSH
- skills paths in plugin.json need ./ prefix

Changed source to "./" (matches Anthropic's own marketplace pattern)
and simplified skills to "./skills" (auto-discovers all skill dirs).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.claude-plugin/marketplace.json

@@ -13,10 +13,7 @@
     {
       "name": "marketing-skills",
       "description": "38 marketing skills for technical marketers and founders: ASO, CRO, copywriting, cold email, SEO, AI SEO, paid ads, ad creative, churn prevention, pricing strategy, referral programs, revenue operations, sales enablement, customer research, site architecture, and more",
-      "source": {
-        "source": "github",
-        "repo": "coreyhaines31/marketingskills"
-      }
+      "source": "./"
     }
   ]
 }