ci: Bump actions/checkout from 4 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 4 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v4...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

@@ -16,7 +16,7 @@ jobs:
     name: Build & Format Check
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Instruction Sync (Claude/Codex)
         run: bash scripts/check-instructions-sync.sh
@@ -40,7 +40,7 @@ jobs:
     runs-on: ubuntu-latest
     needs: build
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Setup .NET
         uses: actions/setup-dotnet@v4
@@ -77,7 +77,7 @@ jobs:
         image: docker:dind
         options: --privileged
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Setup .NET
         uses: actions/setup-dotnet@v4
@@ -111,7 +111,7 @@ jobs:
     runs-on: ubuntu-latest
     needs: [test-unit, test-integration]
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Download Unit Test Results
         uses: actions/download-artifact@v4
@@ -178,7 +178,7 @@ jobs:
     runs-on: ubuntu-latest
     needs: build
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Setup .NET
         uses: actions/setup-dotnet@v4

.github/workflows/codeql.yml

@@ -25,7 +25,7 @@ jobs:
 
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Setup .NET
         uses: actions/setup-dotnet@v4