fix(plugins): plugin.json repository field must be a string

Claude Code's manifest schema requires repository as a URL string, not the
{type,url} object form — the marketplace install failed validation
("expected string, received object") the first time it was tried against a
freshly pushed marketplace.

.claude-plugin/plugin.json

@@ -6,10 +6,7 @@
     "url": "https://git.smartgift.io.vn/Smartgift-AI"
   },
   "homepage": "https://git.smartgift.io.vn/Smartgift-AI/Claude-Code-Monitor",
-  "repository": {
-    "type": "git",
-    "url": "https://git.smartgift.io.vn/Smartgift-AI/Claude-Code-Monitor.git"
-  },
+  "repository": "https://git.smartgift.io.vn/Smartgift-AI/Claude-Code-Monitor.git",
   "license": "MIT",
   "keywords": ["dashboard", "monitoring", "sessions", "cost", "lanes", "mcp", "claude-code"],
   "commands": [