DOCA upgrade

Where to start: This skill is the bundle's single source of truth for the discipline that wraps a DOCA upgrade or downgrade. Open TASKS.md when the user wants to do something with an upgrade (detect the gap, ask for confirmation, walk the guided upgrade, verify it, or diagnose a failed one); open CAPABILITIES.md when the question is what does an upgrade even cover (the host apt-upgrade vs BFB reflash vs container-tag bump vs downgrade/rollback modes, the never-auto rule, the confirmation gate, the sunset-awareness concern, the error taxonomy, and the upgrade-specific safety overlay).