feat(upgrade): implement end-to-end upgrade command

[dipto0321]

Phase 4: Upgrade command + UI

Implement the full upgrade flow:

• detect -> resolve -> fetch versions -> diff -> snapshot -> install -> migrate -> cleanup
• Wire the orchestrator in internal/update/update.go
• huh prompts for interactive manager selection
• --dry-run mode to show plan without changes
• Install/Uninstall via manager interface
• Optional cleanup of old versions via --prune flag

See .puku/plans/nodeup_v1_build_plan_3481ccad.plan.md Phase 4.