fix: serialize preview-dispatch per PR to preserve event order

A PR's events fire as independent workflow runs, so a push (synchronize) and a
close could be dispatched out of order. The cloud handler's concurrency
collapses by branch but can't fix arrival order — an update reaching it while a
destroy is in-flight cancels the destroy and leaks the preview. Add a per-PR
concurrency group (cancel-in-progress: false) so events dispatch in order; the
flow is one short API call, so queuing is cheap.

Addresses the Devin review note on overlapping dispatches.

Author: myftija

.github/workflows/preview-dispatch.yml

@@ -6,6 +6,15 @@ on:
   pull_request:
     types: [opened, reopened, synchronize, closed, labeled, unlabeled]
 
+# Serialize a PR's events so dispatches arrive in order. Cloud-side concurrency
+# collapses by branch but can't fix out-of-order arrival — e.g. a push racing a
+# close could cancel the in-flight destroy and leak the preview. One short API
+# call, so queuing is cheap; cancel-in-progress: false lets an in-flight
+# dispatch finish (GitHub keeps only the latest pending, the desired behavior).
+concurrency:
+  group: preview-dispatch-${{ github.event.pull_request.number }}
+  cancel-in-progress: false
+
 permissions: {}
 
 jobs: