engine: agents.resumable is written on every via-node registration but never read

[willwashburn]

Severity: minor (no-op) · packages/engine/src/db/schema.ts:75, written at engine/node.ts:1168 and :1182

The resumable column is persisted from agent.register but has zero read sites anywhere in engine/, routes/, or the adapters (only the schema definition and the two writes match). A stored flag that is never acted on.

Fix: wire it into resume behavior or drop the column.

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Found during a multi-agent reliability review of the relay tool/protocol.