Migrate devtools/pkg/ghtest and ghauth from devtools/pkg/executor to go-cmdexec

[jaeyeom]

Summary

devtools/pkg/executor has been replaced by github.com/jaeyeom/go-cmdexec across the codebase (20+ files migrated). Three packages still import the old executor:

• devtools/pkg/ghtest (mock.go, mock_test.go, skip.go)
• devtools/pkg/ghauth (ghcli.go, ghcli_test.go)

Tasks

• Migrate devtools/pkg/ghtest to use go-cmdexec
• Migrate devtools/pkg/ghauth to use go-cmdexec
• Delete devtools/pkg/executor package entirely
• Remove stale FIXME/TODO comments that referenced the old executor

Context

The old executor package has open FIXME comments (e.g., no-op cancel func, gosec G118) that are not worth fixing since the package should be removed.