[Epic] Replication opt-in/out & directional-control API design

Tracking epic for how operators express replication direction/opt-in-out: accepting {sends, receives} on add_node/set_node, explicit node isolation (containment mode), and making replication opt-out by default.

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_Filed by Claude (Opus 4.8), issue-backlog triage & grouping pass._

No files, tests, or entry points are named. Start by locating the add_node and set_node APIs and the replication controls they expose, then review how directional sends/receives, containment mode, and opt-out defaults should fit together; done means the API design and intended replication behavior are agreed and documented.

Written by the indexing model from the issue text.