ci: pin all actions to verified SHAs and clear zizmor findings

Pin every action to a commit SHA whose tag comment matches (verified via gh api),
add least-privilege permissions, set persist-credentials: false, and replace the
archived actions-rs/cargo with a plain cargo test. zizmor clean at default persona.

.github/workflows/stale.yml

@@ -3,6 +3,9 @@ on:
   schedule:
     - cron: "30 1 * * *"
 
+permissions:
+  contents: read
+
 jobs:
   close-issues:
     runs-on: ubuntu-latest
@@ -10,7 +13,7 @@ jobs:
       issues: write
       pull-requests: write
     steps:
-      - uses: actions/stale@v4.1.1
+      - uses: actions/stale@a20b814fb01b71def3bd6f56e7494d667ddf28da # v4.1.1
         with:
           days-before-issue-stale: 30
           days-before-issue-close: 7