fix(the-workshop): add signals-dashboard preview.png + logo so the canvas validates

The signals-dashboard extension was missing its required screenshot asset, so it failed awesome-copilot's validateExtensionManifest (logo must equal "assets/preview.png" and the file must exist) and never materialized -- meaning the Cairn canvas would not ship to the GHCP app. Adds the 1024x1024 preview.png and the convention logo field. Validated locally: node eng/validate-plugins.mjs -> extension signals-dashboard is valid; all 70 plugins + 19 extensions pass.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot-Session: 26faf13e-639c-4a21-ac05-c0dc2bff7c62

extensions/signals-dashboard/.github/plugin/plugin.json

@@ -13,5 +13,6 @@
     "coordination",
     "canvas"
   ],
+  "logo": "assets/preview.png",
   "extensions": "."
 }