ci: declare workflow-level contents: read on 3 workflows

Pins the default GITHUB_TOKEN to contents: read on the workflows in
.github/workflows/ that don't call a GitHub API beyond the initial
checkout. The other workflows in this directory are left implicit
because they need write scopes that a maintainer is better placed
to declare.

Motivation: CVE-2025-30066 (March 2025 tj-actions/changed-files
compromise) exfiltrated GITHUB_TOKEN from workflow logs. Per-workflow
caps bound runtime authority irrespective of repo or org default,
give drift protection if the default ever widens, and are credited
per-file by the OpenSSF Scorecard Token-Permissions check.

YAML validated locally with yaml.safe_load.

Signed-off-by: Arpit Jain <arpitjain099@gmail.com>

Author: arpitjain099

.github/workflows/ci.yml

@@ -2,6 +2,9 @@ name: CI
 
 on: [push, pull_request]
 
+permissions:
+  contents: read
+
 jobs:
   lint:
     runs-on: ubuntu-latest

.github/workflows/indexing_top_gems.yml

@@ -2,6 +2,9 @@ name: Index top 100 gems
 
 on: [push, pull_request]
 
+permissions:
+  contents: read
+
 jobs:
   indexing_top_gems:
     runs-on: ubuntu-latest

.github/workflows/memleak.yml

@@ -12,6 +12,9 @@ on:
       - "rust/**"
       - "ext/**"
 
+permissions:
+  contents: read
+
 jobs:
   memleak:
     runs-on: ubuntu-latest