Align release packaging with dmgpio to prevent empty release archives

[Copilot]

Release artifacts in dmuart were being published as effectively empty packages. This PR aligns the release packaging behavior with dmgpio so archives are built from real build outputs, not fallback-created directories.

• Problem scope

  • release.yml created package directories during archive assembly, which allowed zipping placeholder content when expected build-generated package directories were missing.

• Change made

  • Removed directory creation in the release archive step:
    • mkdir -p "$BUILD_DIR/packages/dmuart"
    • mkdir -p "$BUILD_DIR/packages/dmuart_port"
  • Packaging now assumes and uses the package directories produced by the build pipeline, matching dmgpio’s release workflow contract.

• Resulting behavior

  • Release ZIPs are generated from actual package contents under build_$BUILD_NAME/packages/..., avoiding silent creation of minimal/empty archives.

# before
mkdir -p "$BUILD_DIR/packages/dmuart"
echo "${{ github.event.release.body }}" > "$BUILD_DIR/packages/dmuart/RELEASE_NOTES.txt"

# after
echo "${{ github.event.release.body }}" > "$BUILD_DIR/packages/dmuart/RELEASE_NOTES.txt"