ci: escape '+' in the probe branch push filter

'+' is a glob metacharacter in GitHub branch filters, so the literal
branch name never matched and the push trigger didn't fire.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: nikhilunni

.github/workflows/nbd-probe.yml

@@ -30,7 +30,8 @@ on:
   # merging — the workflow is meant to be manual-only.
   push:
     branches:
-      - worktree-ci+nbd-blacksmith-probe
+      # NB: '+' is a glob metachar in GitHub branch filters, escape it.
+      - worktree-ci\+nbd-blacksmith-probe
 
 permissions:
   contents: read