refactor: modernize Go codebase for Go 1.25

- Replace deprecated io/ioutil with os.ReadFile, os.WriteFile, etc.
- Replace interface{} with any
- Replace github.com/pkg/errors with stdlib fmt.Errorf %w wrapping
- Use errors.As() instead of direct type assertions on errors
- Use strings.Cut() for delimiter parsing
- Use slices.Contains() for linear searches
- Use t.Setenv() and t.TempDir() in tests
- Update CI workflows to Go 1.25

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -25,7 +25,7 @@ jobs:
     - name: Setup Go
       uses: actions/setup-go@v6
       with:
-        go-version: '1.22'
+        go-version: '1.25'
     - id: go-cache-paths
       run: |
         echo "::set-output name=go-build::$(go env GOCACHE)"

.github/workflows/release.yml

@@ -29,7 +29,7 @@ jobs:
     - name: Setup Go
       uses: actions/setup-go@v6
       with:
-        go-version: '1.22'
+        go-version: '1.25'
     - name: Install Snapcraft
       uses: samuelmeuli/action-snapcraft@v3
     - name: Setup Snapcraft