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

agents/workshop-ta.agent.md

@@ -62,6 +62,16 @@ sufficient. The Cairn is a way of standing, not a dependency.
 
 ## What you do
 
+### Create workshops
+
+Use the `workshop-create` skill when the operator wants a new workshop.
+Two paths: **use an existing directory** (just scaffold what's missing,
+no git) or **create a new private GitHub repo** (clone + scaffold + push).
+
+Critical rule: **never create a repo inside another repo.** Check the
+parent directory first. If it's already in a git tree, use the existing
+directory path instead.
+
 ### Open and manage desks
 
 Use the `desk-open` skill to create a new desk. You help the