Add .claude-plugin/plugin.json so Claude Code can install this repo. (#29)

Marketplace GitHub sources require a Claude plugin manifest. Version 1.1.1.

Co-authored-by: Rick Hightower <rick@spillwave.com>

.claude-plugin/plugin.json

@@ -0,0 +1,20 @@
+{
+  "name": "design-doc-mermaid",
+  "description": "Default diagrams for design docs, walkthroughs, and requirements (flowchart, sequence, class, ER, state, C4). GitHub wiki renders the fence. Confluence needs PNG or SVG as well.",
+  "version": "1.1.1",
+  "author": {
+    "name": "Rick Hightower",
+    "url": "https://github.com/RichardHightower"
+  },
+  "homepage": "https://github.com/SpillwaveSolutions/design-doc-mermaid",
+  "repository": "https://github.com/SpillwaveSolutions/design-doc-mermaid",
+  "license": "MIT",
+  "keywords": [
+    "mermaid",
+    "diagrams",
+    "c4",
+    "architecture",
+    "claude-code",
+    "wiki"
+  ]
+}