Add entity merge and split contract

[haasonsaas]

Summary\n- add MergeEntities and SplitEntity RPCs to entities.v1.EntityService\n- add ref-key, merge response, and split response messages for canonical lifecycle changes\n- regenerate Go, Python, and TypeScript bindings\n\n## Testing\n- buf generate --path proto/entities/v1/entities.proto --timeout 10m\n- buf lint --path proto/entities/v1/entities.proto\n- go test ./... -count=1\n- npm run check:package\n\nPartially addresses evalops/entities#11.

[cursor]

PR Summary

Medium Risk
Adds new RPCs and message types to a public API contract and regenerates multi-language bindings, which can impact client/server compatibility if versions drift or implementations lag.

Overview
Extends entities.v1.EntityService with two new unary RPCs: MergeEntities and SplitEntity, enabling canonical-entity lifecycle operations at the API level.

Introduces supporting proto messages (CrossSystemRefKey, MergeEntitiesRequest/Response, SplitEntityRequest/Response) and regenerates the Go, Connect-Go, Python, and TypeScript client/server stubs to include the new methods and types.

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