Modernize the codebase to use idiomatic Go 1.25 patterns, removing deprecated APIs and reducing external dependencies.
- Replace deprecated `io/ioutil` with `os.ReadFile`, `os.WriteFile`, `os.MkdirTemp`, `os.CreateTemp`
- Replace `interface{}` with `any` (Go 1.18+)
- Remove `github.com/pkg/errors` dependency entirely, using stdlib `fmt.Errorf` with `%w` and `errors.New`
- Use `errors.As()` instead of direct type assertions on error values
- Use `strings.Cut()` for delimiter parsing instead of `strings.Split` + length check
- Use `slices.Contains()` for linear search in `ContextExists()`
- Use `t.Setenv()` and `t.TempDir()` in tests instead of manual env save/restore and temp dir cleanup
- Delete unused `internal/testutil/tempfile.go` helper
- Update GitHub Actions CI and release workflows from Go 1.22 to 1.25
Net result: -70 lines, 1 fewer external dependency.
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
