[M1] CI/CD pipeline and Docker image

[Fosurero]

Description

Set up GitHub Actions CI and multi-stage Docker build.

Tasks

• GitHub Actions workflow: check, clippy, fmt, test, release build
• Demo smoke test in CI
• Upload binary artifact
• Multi-stage Dockerfile (rust:1.86 → debian:bookworm-slim)

Status: ✅ Complete – M1 PoC