feat: add Li+ icon to .mcpb manifest (#167)

Add icon.png (128x128 PNG) and reference it in manifest.json
so MCP directories and client UIs can display the brand icon.

Closes #166

Co-authored-by: liplus-lin-lay <liplus-lin-lay@users.noreply.github.com>

Author: liplus-lin-lay

mcp-server/icon.png

@@ -17,6 +17,7 @@
   "documentation": "https://github.com/Liplus-Project/github-webhook-mcp#readme",
   "support": "https://github.com/Liplus-Project/github-webhook-mcp/discussions",
   "license": "MIT",
+  "icon": "icon.png",
   "server": {
     "type": "node",
     "entry_point": "server/index.js",