fix(the-workshop): add HTTP error boundary and make workshop detection non-terminal

Two follow-up review findings:
- signals-dashboard: the async createServer callback had no top-level
  error boundary, so a malformed %-encoded path, a stash write on a
  read-only workshop, or a scan failure rejected a promise the server
  never awaits — hanging the request and risking an unhandled-rejection
  crash. Wrap the handler and return a controlled 500.
- workshop-create: Path A treated finding a marker (desks/, CAIRN.md,
  etc.) as 'just use it', an early stop that left partially initialized
  workshops incomplete. Make it detection-only and continue scaffolding
  whatever is missing, per the 'only add what's missing' principle.

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

skills/workshop-create/SKILL.md

@@ -21,9 +21,11 @@ The operator already has a folder they want to use. Maybe it's a repo
 they cloned, maybe it's a local project folder.
 
 1. **Confirm the path exists.** If not, ask the operator for a valid path.
-2. **Check if it's already a workshop.** Look for `desks/` or `classroom/`
-   folders, a `workshop.md`, `CAIRN.md`, or `hands-up.md`. If any exist,
-   it's already a workshop — just use it.
+2. **Detect existing workshop markers.** Look for `desks/` or `classroom/`
+   folders, a `workshop.md`, `CAIRN.md`, or `hands-up.md`. Finding any of
+   these tells you this is an existing workshop — but this is detection
+   only, not a stopping point. Continue to the next step and add whatever
+   is missing; never overwrite what is already there.
 3. **Scaffold the workshop structure** (only what's missing):
    ```
    <path>/