Add Claude Code plugin + marketplace manifests

Adds .claude-plugin/plugin.json and .claude-plugin/marketplace.json so the repo can be installed via /plugin marketplace add. Uses "skills": ["./"] so the existing root SKILL.md is picked up without relocation — standalone install path is unchanged. Also removes stray Cocoon AI author reference from SKILL.md frontmatter.

.claude-plugin/plugin.json

@@ -0,0 +1,17 @@
+{
+  "name": "diagram-design",
+  "description": "Editorial-quality technical and product diagrams — 13 types rendered as standalone HTML with inline SVG, skinnable to match your brand",
+  "version": "1.0.0",
+  "homepage": "https://github.com/cathrynlavery/diagram-design",
+  "repository": "https://github.com/cathrynlavery/diagram-design",
+  "license": "MIT",
+  "keywords": [
+    "diagrams",
+    "svg",
+    "architecture",
+    "flowchart",
+    "visualization",
+    "editorial"
+  ],
+  "skills": ["./"]
+}