fix(ship): exclude current PR from queue-awareness (self-reference bug)

Version gate flagged PR #1168 as stale because the util counted the PR
itself as a queued claim. The exclude filter removes that self-reference.

New --exclude-pr <N> flag on bin/gstack-next-version. CI workflows pass
github.event.pull_request.number / CI_MERGE_REQUEST_IID. Local /ship
auto-detects via gh pr view when the flag isn't passed, with a warning
recording the auto-exclusion so it's observable.

Caught during the first live ship through the v1.8.0.0 gate — the kind
of dogfood the whole release is designed for.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/version-gate.yml

@@ -58,6 +58,7 @@ jobs:
             --bump "${{ steps.bump.outputs.level }}" \
             --current-version "${{ steps.versions.outputs.base_version }}" \
             --workspace-root null \
+            --exclude-pr "${{ github.event.pull_request.number }}" \
             > next.json 2> next.err
           RC=$?
           if [ "$RC" != "0" ] || [ ! -s next.json ]; then