refactor(proto): split schema by domain

[connorcarpenter15]

Summary

• split the openengine.v1 schema into domain-oriented proto files
• retain one openengine.v1.OpenEngine service and one protobuf package
• keep openengine.proto as the service entry point
• add a package README and update documentation and code-generation guidance

Why

The schema had grown to 708 lines, 79 message and enum types, and 14 RPCs across generation, discovery, lifecycle, LoRA, KV, and observability domains. Domain files make the contract easier to navigate and evolve without introducing new protobuf packages or services.

Impact

All message, enum, service, and RPC descriptors are unchanged. Generated source now spans domain-specific modules, so consumers must compile every .proto file under proto/openengine/v1/ rather than compiling only openengine.proto.

Validation

• buf build
• buf lint
• buf format --diff --exit-code
• descriptor equivalence against main: 80 top-level symbols matched
• Python binding generation and OpenEngineStub import: 20 generated modules
• markdownlint-cli2 0.23.0: 0 errors
• Lychee 0.24.2: 67 links checked, 0 errors