build(deps): bump actions/checkout from 5 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 5 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v5...v6)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

.github/workflows/ci.yml

@@ -19,7 +19,7 @@ jobs:
     name: macOS (Swift 6.2, debug)
     runs-on: macos-26
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       - name: Select Xcode 26.0
         run: sudo xcode-select -s /Applications/Xcode_26.0.app
@@ -48,7 +48,7 @@ jobs:
     runs-on: ubuntu-latest
     container: swift:6.2
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       - name: Cache Swift packages
         uses: actions/cache@v4
@@ -67,7 +67,7 @@ jobs:
     runs-on: ubuntu-latest
     container: swift:6.0
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       - name: Cache Swift packages
         uses: actions/cache@v4

.github/workflows/swift-format.yml

@@ -17,7 +17,7 @@ jobs:
     permissions:
       contents: write
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       - name: Cache swift-format
         id: cache-swift-format

.github/workflows/swiftlint.yml

@@ -18,7 +18,7 @@ jobs:
     runs-on: ubuntu-latest
     container: swift:6.2
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       - name: Restore swiftlint cache
         id: cache-swiftlint-restore