fix(pm-skills): use plain tag name for ref, not refs/tags/ (#1771)

* 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: github/awesome-copilot#1767 / github/awesome-copilot#1770

* fix: regenerate marketplace.json after pm-skills ref fix

The previous force-push (7fbf75a) updated plugins/external.json only.
.github/plugin/marketplace.json is a derived artifact regenerated by
`npm start` and must be kept in sync. The Validate README.md workflow
detected the mismatch and failed.

Ran `npm ci && npm start` locally; produces a single 1-line update to
marketplace.json matching the plugins/external.json change.

.github/plugin/marketplace.json

@@ -603,7 +603,7 @@
       "source": {
         "source": "github",
         "repo": "Avyayalaya/pm-skills-arsenal",
-        "ref": "refs/tags/v2.1.0"
+        "ref": "v2.1.0"
       }
     },
     {

plugins/external.json

@@ -316,7 +316,7 @@
     "source": {
       "source": "github",
       "repo": "Avyayalaya/pm-skills-arsenal",
-      "ref": "refs/tags/v2.1.0"
+      "ref": "v2.1.0"
     }
   },
   {