chore: add MIT license and go.mod to gkgpb Go submodule

Summary

Splits gkgpb into its own Go submodule with an MIT license so consumers like Workhorse can depend on just the proto stubs without pulling in the EE-licensed parent module. Workhorse references the submodule by pseudo-version (commit hash).

• gkgpb/go.mod, gkgpb/go.sum, gkgpb/LICENSE (MIT)
• check-go-proto CI job updated to build from submodule directory
• proto-go.sh runs go mod tidy in submodule after generation

Prior art

• ai-assist!2760 - same pattern for the Duo Workflow Service Go proto client

Related

• gitlab!229394 - Workhorse orbit query acceleration (imports gkgpb)
• omnibus-gitlab!9278 - omnibus license allowlist entry (defense-in-depth)
• #349 (closed)
• #348 (closed)