venture-loop.yml: quote the step name with a colon in it (YAML read it as a key: a jobless failed run)

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: open-workflows-venture-loop

.github/workflows/venture-loop.yml

@@ -79,5 +79,5 @@ jobs:
           done
       - name: launch a venture held for its batch
         run: python3 .github/time/venture.py stage "$card" ready
-      - name: measure (7 days on: signups against the plan's lines)
+      - name: "measure (7 days on: signups against the plan's lines)"
         run: python3 .github/time/venture.py stage "$card" launched