Pin GitHub Actions to commit SHAs (#270)

Pins GitHub Actions `uses:` references in `github/accessibility-scanner`
to immutable commit SHAs.

## Summary

| Metric | Count |
| --- | ---: |
| Files changed | 3 |
| Files scanned | 2 |
| Refs found | 3 |
| Refs pinned | 3 |
| Skipped refs | 0 |
| Warnings | 0 |
| Errors | 0 |

## Why

Pinning actions to full commit SHAs prevents future tag or branch
retargeting from changing workflow behavior without review.

## Reviewer notes

- Original refs are preserved in inline comments when possible.
- Pin comments use the Dependabot-compatible original-ref style.
- Branch refs were allowed and pinned to their current HEAD; review
mutable-branch pins carefully.
- No minimum action age was enforced for this run.

## Pinned refs

| Location | Before | After | Resolved as |
| --- | --- | --- | --- |
| `.github/workflows/lint.yml:26` | `actions/checkout@v7` |
`actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1` | `tag` |
| `.github/workflows/lint.yml:29` | `actions/setup-node@v7` |
`actions/setup-node@820762786026740c76f36085b0efc47a31fe5020` | `tag` |
| `.github/workflows/test.yml:34` | `actions/checkout@v7` |
`actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1` | `tag` |

## Dependabot

- Added a 7-day cooldown (`cooldown: default-days: 7`) to the existing
`github-actions` Dependabot configuration.
- The cooldown delays applying a newly published action release for 7
days, reducing exposure to a compromised or broken release while keeping
you SHA-pinned.

---

Generated by pinner 0.1.0.

Author: github-security-bot

.github/dependabot.yml

@@ -36,6 +36,8 @@ updates:
           - "*"
     schedule:
       interval: "weekly"
+    cooldown:
+      default-days: 7
   - package-ecosystem: "bundler"
     directory: "/sites/site-with-errors"
     groups:

.github/workflows/lint.yml

@@ -23,10 +23,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v7
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
 
       - name: Setup Node
-        uses: actions/setup-node@v7
+        uses: actions/setup-node@820762786026740c76f36085b0efc47a31fe5020 # v7.0.0
         with:
           node-version: 24
           cache: npm

.github/workflows/test.yml

@@ -31,7 +31,7 @@ jobs:
         site: ['sites/site-with-errors']
     steps:
       - name: Checkout
-        uses: actions/checkout@v7
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
 
       - name: Setup Ruby
         uses: ruby/setup-ruby@95ef2b042f9d7a56d8268cba8559e2842e2ad01b