Clear the staging directory before assembling into it

roadie's workspace persists between jobs, so yesterday's aborted run
left stage/2026-09-17-2 behind and today's run found two dated
directories where it expected one. The guard caught it and refused to
guess, which is the right outcome, but the tree should not have been
there to find.

The merged assemble job begins with a checkout, which cleans the
workspace as a side effect, and this does not depend on that.

.github/workflows/snapshot.yml

@@ -156,6 +156,13 @@ jobs:
           # and the snapshots tree are laid out. The docs/macos/pip/source/
           # windows subdivision belongs to a published release, and it is the
           # promotion step that sorts files into it on the way there.
+          # Explicitly, rather than trusting the checkout to have cleaned it.
+          # This runs on a self-hosted runner whose workspace persists between
+          # jobs, and a previous run's tree left behind here is indistinguishable
+          # from this one's until something downstream has to choose between
+          # them.
+          rm -rf stage
+
           STAGE="stage/${{ needs.location.outputs.name }}"
           mkdir -p "${STAGE}/yum"