feat: add workshop-create skill — two paths, never nest repos

Syncs with jennyf19/the-workshop PR #8 (merged). Adds the
workshop-create skill with Path A (existing dir) and Path B
(new GitHub repo), explicit guard against repo-in-repo nesting.
TA agent updated with workshop-create section.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 26faf13e-639c-4a21-ac05-c0dc2bff7c62

plugins/the-workshop/.github/plugin/plugin.json

@@ -22,6 +22,7 @@
     "./skills/bench-read/",
     "./skills/desk-journal/",
     "./skills/desk-open/",
-    "./skills/signal-write/"
+    "./skills/signal-write/",
+    "./skills/workshop-create/"
   ]
 }