Add binary release artifacts to release workflow

[dorser]

Summary

The README mentions running dist/micromize-linux-[amd64|arm64] binaries, but the release workflow only pushes Docker images. No pre-built binaries are attached to GitHub releases.

Current State

The Makefile builds binaries:

cd build/src && GOOS=linux GOARCH=$@ CGO_ENABLED=0 go build -tags release -ldflags "$(LDFLAGS)" -o ../../$(OUTPUT_DIR)/micromize-linux-$@ ./cmd/micromize

But .github/workflows/release.yml only has Docker image push.

What's Needed

Add a release-binaries job to the release workflow that:

1. Builds static Linux binaries for amd64 and arm64
2. Generates checksums (SHA256)
3. Uploads binaries + checksums as release assets

Alternatives

Consider using GoReleaser which handles binary builds, checksums, SBOMs, and release asset uploads in one step. A separate issue tracks adding .goreleaser.yml.