ci(unstable): one rolling pre-release, one publisher

Two pushes in quick succession started two Unstable runs, and both ran
`gh release delete unstable --cleanup-tag` before republishing. The slower one
found the tag moved under it and failed at the publish step with its APK already
built — a red run that said nothing about the code, which is the worst kind.

A concurrency group cancels the older run instead, so the newest commit is always
the one that reaches the pre-release.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: malys

.github/workflows/unstable.yml

@@ -7,6 +7,14 @@ on:
     branches: [master]
   workflow_dispatch:
 
+# One rolling pre-release means one publisher. Two pushes in quick succession raced on
+# deleting and recreating the "unstable" tag, and the loser failed at the publish step
+# with its APK already built — a red run that said nothing about the code. The older run
+# is cancelled instead, so the newest commit is always the one that gets published.
+concurrency:
+  group: unstable-${{ github.ref }}
+  cancel-in-progress: true
+
 permissions:
   contents: write