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.

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 with 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

@@ -335,7 +335,7 @@
     "source": {
       "source": "github",
       "repo": "Avyayalaya/pm-skills-arsenal",
-      "ref": "refs/tags/v2.1.0"
+      "ref": "v2.1.0"
     }
   },
   {