Add check_required_commands() function to verify that all required
external tools are available before the script begins execution:
- docker: Container runtime
- compose: Docker compose wrapper (0k-scripts)
- sudo: Required for filestore operations
Benefits:
- Fails fast with a clear error message listing missing commands
- Prevents cryptic 'command not found' errors mid-execution
- Documents script dependencies explicitly
- Called immediately after argument validation in upgrade.sh
00c12769bc