fix: migrate from deprecated io/ioutil and enhance Workbench compatib…

[kjw3]

…ility tests

Major Changes:

• Replace io/ioutil with io and os packages across 10 files (14 function calls)
  • ioutil.ReadAll -> io.ReadAll
  • ioutil.ReadFile -> os.ReadFile
  • ioutil.WriteFile -> os.WriteFile
  • ioutil.ReadDir -> os.ReadDir
• Remove duplicate start command registration in pkg/cmd/cmd.go

Workbench Compatibility Enhancements:

• Add comprehensive documentation to cli_output_compatibility_test.go
• Add Test_ListWithOrgFlag to verify 'brev ls --org' support
• Add Test_ShortHelpFlag to verify 'brev -h' works (Workbench uses this)
• Add Test_VersionWithNoCheckLatestFlag for flag variation testing
• Add Test_InstanceListColumnHeadersStability for critical column validation
• Add Test_OrgListColumnHeadersStability for org list format
• Add Test_CommandExistenceForWorkbench for comprehensive command checks
• Improve Test_StartCommandFormat to explicitly verify --org flag

All changes verified with successful build and test execution. Tests ensure backward compatibility with NVIDIA AI Workbench integration.