ci: give the test, perf guard and branch image workflows a read-only token

.github/workflows/docker-publish.yml

@@ -21,6 +21,10 @@ on:
       - ".github/workflows/docker-publish.yml"
   workflow_dispatch:
 
+# Jobs that need more (the registry push, the attestation) say so themselves.
+permissions:
+  contents: read
+
 jobs:
   prepare:
     name: Prepare

.github/workflows/perf-guard.yml

@@ -9,6 +9,11 @@ on:
   push:
   pull_request:
 
+# The job builds and runs the checked-out code; a compromised step must not
+# hold a token that can write to the repository.
+permissions:
+  contents: read
+
 jobs:
   perf-guard:
     name: Perf guard

.github/workflows/tests.yml

@@ -4,6 +4,11 @@ on:
   push:
   pull_request:
 
+# The job builds and runs the checked-out code; a compromised step must not
+# hold a token that can write to the repository.
+permissions:
+  contents: read
+
 jobs:
   vitest:
     name: Vitest