Copy a Datahike store into a NEW store with `:keep-history? false`.

Datahike validates the connect-time config against the stored one, so
`:keep-history?` cannot be changed in place — a store created with history
refuses to open under a config that disables it. The only route is a copy.

Contract:
  (plan source-cfg target-cfg)  => the migration plan, no I/O beyond a read
  (migrate! opts)               => Result<report>

`migrate!` NEVER writes to, deletes, or reconfigures the source. It refuses
to run without `:i-mean-it true`, and refuses a target that already exists.

Datahike implementation of hive-spi.kg.protocol/IKGStore + IPersistentKGStore
+ ITemporalKGStore. Config (db-path/backend/store-id/writer) and the core-norms
classpath resource are injected by the host — the store resolves no env/config
of its own and carries no hive-mcp dependency.