fix(ci): download builddeck.tag from release_build step for reliable release handoff (#82)

The release step always got a fresh workspace, so the local
builddeck.tag file from release_build was never available. The
release step always skipped with 'No tag was created'.

Fix:
- release_build now re-uploads builddeck.tag (downloaded from tag
  step) under its own step key, so the artifact is immediately
  available to downstream steps.
- release downloads from --step release_build (the step that just
  ran) instead of checking a local file. Includes the [[ -s
  builddeck.tag ]] content guard to prevent empty-version crashes.
- Conditional upload: builddeck.tag is only uploaded from
  release_build if it exists and has content (i.e., a tag was
  created). 'dev' builds are not uploaded.

Author: alexhraber

.buildkite/pipeline.yml

@@ -104,12 +104,18 @@ steps:
       sha256sum builddeck | tee builddeck.sha256
       buildkite-agent artifact upload "builddeck"
       buildkite-agent artifact upload "builddeck.sha256"
+      # Upload builddeck.tag so the release step can download it from this step
+      # regardless of which agent it runs on. The tag step also uploads it, but
+      # having it from release_build ensures immediate availability.
+      if [[ -s builddeck.tag ]]; then
+        buildkite-agent artifact upload "builddeck.tag"
+      fi
 
   - label: ":rocket: GitHub Release"
     key: release
     depends_on: release_build
     command: |
-      if [[ -s builddeck.tag ]]; then
+      if buildkite-agent artifact download builddeck.tag . --step release_build 2>/dev/null && [[ -s builddeck.tag ]]; then
         VERSION=$(cat builddeck.tag)
         echo "Creating GitHub release for ${VERSION}"
         .buildkite/release.sh "${VERSION}"