feat(devex): Docker image for the gRPC server

[EngineerProjects]

Summary

Provide an official Docker image for the Nexus gRPC server so users can
deploy it without installing Go.

Deliverables

• Dockerfile — multi-stage build (build stage: Go, runtime stage: distroless)
• docker-compose.yml — local dev setup with env vars pre-wired
• GitHub Actions job added to release.yml — builds and pushes image to
  ghcr.io/engineerprojects/nexus-engine on every v* tag

Dockerfile sketch

FROM golang:1.25 AS builder
WORKDIR /src
COPY . .
RUN CGO_ENABLED=0 go build -trimpath -ldflags="-s -w" -o /nexus-grpc ./cmd/grpc

FROM gcr.io/distroless/static-debian12
COPY --from=builder /nexus-grpc /nexus-grpc
EXPOSE 50051
ENTRYPOINT ["/nexus-grpc"]

Acceptance criteria

• docker build . produces a working image
• docker run -e ANTHROPIC_API_KEY=... -p 50051:50051 nexus-engine starts the server
• Image size < 30 MB
• docker-compose up starts server + a simple health-check service
• CI publishes image to GHCR on tag push
• docs/transports.md updated with Docker deployment instructions