feat(worker): one draft per game; a rebuild replaces the earlier one

The draft branch was named after the job. Rerunning a build pushed onto the
earlier run's branch, which had review commits, so the push was rejected
and the finished build failed at its last step. Separate branches per run
would pile up open PRs for the same game instead.

A game now has one draft at a time, on draft/<id>. Before pushing, the
worker closes the game's open draft PRs (draft/<id>, and draft/<id>-<job>
as earlier versions named them) and deletes their branches, then
force-pushes draft/<id> and opens the new PR, which says which PR it
replaces. Other games' drafts and anyone's non-draft branches are never
touched.

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

cmd/gbrgames/worker_test.go

@@ -68,3 +68,48 @@ func TestCheckBotWithRepositoryOnlyToken(t *testing.T) {
 		t.Errorf("identity = %s <%s>", c.botLogin, c.botEmail)
 	}
 }
+
+func TestReplaceOldDraftsClosesOnlyThisGamesDrafts(t *testing.T) {
+	var calls []string
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		calls = append(calls, r.Method+" "+r.URL.Path)
+		switch {
+		case r.Method == http.MethodGet && r.URL.Path == "/api/v1/repos/org/games/pulls":
+			w.Write([]byte(`[
+				{"number": 13, "head": {"ref": "draft/example-4b174b2d"}},
+				{"number": 20, "head": {"ref": "draft/example"}},
+				{"number": 21, "head": {"ref": "draft/example-other"}},
+				{"number": 22, "head": {"ref": "draft/other-4b174b2d"}},
+				{"number": 23, "head": {"ref": "fix/example-ports"}}
+			]`))
+		case r.Method == http.MethodDelete && r.URL.Path == "/api/v1/repos/org/games/branches/draft/example-4b174b2d":
+			w.WriteHeader(http.StatusNoContent)
+		case r.Method == http.MethodDelete:
+			w.WriteHeader(http.StatusNotFound) // already gone is fine
+		default:
+			w.Write([]byte(`{}`))
+		}
+	}))
+	defer srv.Close()
+
+	c := &workerConfig{gitea: srv.URL, repo: "org/games", botToken: "secret"}
+	closed, err := c.replaceOldDrafts("example")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(closed) != 2 || closed[0] != 13 || closed[1] != 20 {
+		t.Errorf("closed %v, want [13 20]: not another game's draft, not draft/example-other, not someone's fix branch", closed)
+	}
+	got := strings.Join(calls, "\n")
+	for _, want := range []string{"PATCH /api/v1/repos/org/games/pulls/13", "PATCH /api/v1/repos/org/games/pulls/20",
+		"DELETE /api/v1/repos/org/games/branches/draft/example-4b174b2d", "DELETE /api/v1/repos/org/games/branches/draft/example"} {
+		if !strings.Contains(got, want) {
+			t.Errorf("missing %q in:\n%s", want, got)
+		}
+	}
+	for _, unwanted := range []string{"pulls/21", "pulls/22", "pulls/23"} {
+		if strings.Contains(got, unwanted) {
+			t.Errorf("touched %s:\n%s", unwanted, got)
+		}
+	}
+}