fix(ci): use self-hosted runner for auto-promote workflow

ubuntu-latest jobs die in ~2s for private repos — org's GitHub-hosted
minute budget is exhausted. The ci.yml already uses self-hosted (Mac mini).
Align the auto-promote workflow to use the same runner.

workflow_dispatch manual runs at 15:06, 15:08, 15:10, 15:22, 15:23 all
failed with the same 2s timeout pattern.

.github/workflows/auto-promote-staging.yml

@@ -31,7 +31,9 @@ permissions:
 
 jobs:
   promote:
-    runs-on: ubuntu-latest
+    # self-hosted Mac mini — org's GitHub-hosted minute budget is exhausted
+    # for private repos; same runner used by ci.yml
+    runs-on: self-hosted
     steps:
       - uses: actions/checkout@v4
         with: