fix(worker): a new branch for every run, so reruns can open their PR

The draft branch was named after the job only. Rerunning a job's build
pushed to the earlier run's branch, which already had review commits, so the
push was rejected and the finished build failed at the very end. Branches
now also carry the run's time.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

cmd/gbrgames/worker.go

@@ -360,7 +360,9 @@ func (c *workerConfig) openPR(dir string, job *builderJob, res *buildResult) (st
 	if len(short) > 8 {
 		short = short[:8]
 	}
-	branch := "draft/" + res.GameID + "-" + short
+	// One branch per run: a rerun of the same job must not push onto the earlier run's branch, which
+	// may have gained review commits since
+	branch := draftBranch(res.GameID, short, time.Now())
 	auth := "http.extraHeader=Authorization: token " + c.botToken
 	steps := []struct {
 		name string
@@ -415,3 +417,7 @@ func readJSON(p string, out any) error {
 	}
 	return json.Unmarshal(raw, out)
 }
+
+func draftBranch(gameID, job string, at time.Time) string {
+	return "draft/" + gameID + "-" + job + "-" + at.UTC().Format("20060102-150405")
+}