docs(plans): add v2 implementation plan with 66 bite-sized tasks
Converts the akka-hosting-alignment design into an executable plan: 12 phases covering branch/scaffold, ConfigDb schema, Commons, Cluster, Security, ControlPlane singletons, Runtime per-node actors, OpcUaServer extraction, AdminUI migration, Host entry point, cleanup, integration tests, deploy scripts, and docs. Each task has files, TDD steps, exact commands, classification, time estimate, and parallelizable list. Co-located .tasks.json drives executing-plans resume from any session.
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