fix(pm-skills): use plain tag name for ref, not refs/tags/...

`apm install pm-skills@awesome-copilot` invokes `git clone --depth=1 --branch=<ref>` against the source repo. `git clone --branch` rejects `refs/tags/v2.1.0` ("Remote branch not found in upstream origin") and only accepts the plain tag name. The fix is a one-character change in the `source.ref` field of the pm-skills entry.

This matches the convention used by the other entries in this file — `chrome-devtools-plugin` uses `chrome-devtools-mcp-v1.0.1`, `ai-ready` uses a SHA. `pm-skills` was the only entry using the `refs/tags/` prefix, inherited from the placeholder example in the external-plugin issue form.

Tested locally: with the fix applied, `apm install pm-skills@awesome-copilot --target claude` resolves cleanly (12 skills + MCP server + 2 hooks integrated).

Original submission: #1767 / #1770

Author: Avyayalaya

plugins/external.json

@@ -1,7 +1,7 @@
 [
   {
     "name": "ai-ready",
-    "description": "Analyze any repository and generate AI-ready configuration — AGENTS.md, copilot-instructions.md, CI workflows, issue templates, and more. Mines your PR review patterns and creates files customized to your stack.",
+    "description": "Analyze any repository and generate AI-ready configuration \u00e2\u20ac\u201d AGENTS.md, copilot-instructions.md, CI workflows, issue templates, and more. Mines your PR review patterns and creates files customized to your stack.",
     "version": "1.0.0",
     "author": {
       "name": "John Papa",
@@ -265,7 +265,7 @@
   },
   {
     "name": "microsoft-events",
-    "description": "Connect your project to Microsoft Build and Ignite sessions — discover relevant talks, explore what's new for your stack, and plan next steps from your development environment.",
+    "description": "Connect your project to Microsoft Build and Ignite sessions \u00e2\u20ac\u201d discover relevant talks, explore what's new for your stack, and plan next steps from your development environment.",
     "version": "1.0.0",
     "author": {
       "name": "Microsoft",
@@ -335,7 +335,7 @@
     "source": {
       "source": "github",
       "repo": "Avyayalaya/pm-skills-arsenal",
-      "ref": "refs/tags/v2.1.0"
+      "ref": "v2.1.0"
     }
   },
   {
@@ -388,7 +388,7 @@
   },
   {
     "name": "whatidid",
-    "description": "Turn your Copilot sessions into proof of impact — research-grounded HTML reports with effort estimation, skills analysis, and ROI metrics from local session logs.",
+    "description": "Turn your Copilot sessions into proof of impact \u00e2\u20ac\u201d research-grounded HTML reports with effort estimation, skills analysis, and ROI metrics from local session logs.",
     "version": "1.0.0",
     "author": {
       "name": "Microsoft",