fix(version-gate): run the head release tool with a bare env, drop outputs it must rewrite, prune the staging worktree

Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

.github/workflows/version-gate.yml

@@ -26,6 +26,9 @@ jobs:
         with:
           fetch-depth: 0
           ref: ${{ github.event.pull_request.head.sha }}
+          # The head's release_tool runs inside the gate (lib/version-gate-train.ts);
+          # no token may sit in the checkout it stages. The base fetch below is public.
+          persist-credentials: false
 
       # The gate's own code comes from the PR's merge commit (github.sha): a PR
       # that edits the gate runs its edit, and an open PR picks up a gate fix