add(plugin): mcp-server-dev — skills for building MCP servers

Three skills guiding developers through MCP server design:
- build-mcp-server: entry-point decision guide (remote HTTP vs MCPB vs local)
- build-mcp-app: interactive UI widgets rendered in chat
- build-mcpb: bundled local servers with runtime

Includes reference files for scaffolds, tool design, auth (DCR/CIMD),
widget templates, manifest schema, and local security hardening.

plugins/mcp-server-dev/.claude-plugin/plugin.json

@@ -0,0 +1,8 @@
+{
+  "name": "mcp-server-dev",
+  "description": "Skills for designing and building MCP servers that work seamlessly with Claude — guides you through deployment models (remote HTTP, MCPB, local), tool design patterns, auth, and interactive MCP apps.",
+  "author": {
+    "name": "Anthropic",
+    "email": "support@anthropic.com"
+  }
+}