Add external plugin cache-stats (#2931)

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

plugins/external.json

@@ -182,6 +182,33 @@
       "sha": "8a8a102ff3ab8736f015764207cdb7e68fcd5ecf"
     }
   },
+  {
+    "name": "cache-stats",
+    "description": "Shows live prompt-cache reuse, expiry, token composition, and AIU cost for the current GitHub Copilot session without making model calls.",
+    "version": "1.0.1",
+    "author": {
+      "name": "Arafat Tehsin",
+      "url": "https://github.com/arafattehsin"
+    },
+    "repository": "https://github.com/arafattehsin/cache-stats-extension",
+    "homepage": "https://github.com/arafattehsin/cache-stats-extension",
+    "license": "MIT",
+    "keywords": [
+      "aiu",
+      "cache",
+      "canvas",
+      "copilot",
+      "cost-visibility",
+      "prompt-cache",
+      "token-usage"
+    ],
+    "source": {
+      "source": "github",
+      "repo": "arafattehsin/cache-stats-extension",
+      "ref": "v1.0.1",
+      "sha": "678c3468977889d5c50a0a5f33898b8d9638523a"
+    }
+  },
   {
     "name": "catalyst-by-zoho",
     "description": "Official GitHub Copilot plugin for Catalyst by Zoho — a full-stack serverless cloud platform. Includes skills covering services, SDKs, CLI workflows, architecture patterns, pricing, migration guides, and Zoho MCP resource management.",