feat: opencode zen adapter + first-run config generation, fix stats/stream bugs

- adapters/opencode.lua: opencode.ai zen free pool adapter — sends the
  opencode client User-Agent (zen fingerprints clients by UA; non-official
  clients hit FreeUsageLimitError); pairs with api_key: public
- config: no config file ships in the repo; first run generates a default
  config at the -config path with a random admin key, loopback listen and a
  keyless zen source (config.EnsureDefault); remove config.example.yaml
- lua: seed bundled adapters from the embedded FS instead of a hardcoded
  name list
- ui: widen model kind select (chat was clipped to 'cha')
- phase 5 bugfixes: stats ms/s bucket mixing, cleanScopes nil, ctx.Err
  guards, direct-path ModelAvailable, empty stream body failure,
  bestImageModel rewrite, transform failure recording, Core.mu, timer,
  effective model for tool-calls

internal/provider/provider_test.go

@@ -298,3 +298,106 @@ func TestChatBusyFailsFast(t *testing.T) {
 		t.Fatalf("first chat: %v", err)
 	}
 }
+
+func eventually(t *testing.T, timeout time.Duration, cond func() bool, msg string) {
+	t.Helper()
+	deadline := time.Now().Add(timeout)
+	for time.Now().Before(deadline) {
+		if cond() {
+			return
+		}
+		time.Sleep(10 * time.Millisecond)
+	}
+	t.Fatalf("timed out: %s", msg)
+}
+
+// TestChatClientCancelNotRecorded: a client disconnect before the response is
+// neither success nor failure — the (source, model) state must stay clean.
+func TestChatClientCancelNotRecorded(t *testing.T) {
+	started := make(chan struct{})
+	release := make(chan struct{})
+	up := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		close(started)
+		<-release // hold the upstream open; release at cleanup
+		fmt.Fprint(w, `{"choices":[{"message":{"content":"late"}}]}`)
+	}))
+	defer up.Close()
+	p := newTestProvider(t, src("mock", up.URL, "openai", "m"))
+	ctx, cancel := context.WithCancel(context.Background())
+	errCh := make(chan error, 1)
+	go func() {
+		_, err := p.Chat(ctx, &types.ChatRequest{
+			Model:    "m",
+			Messages: []types.ChatMessage{{Role: "user", Content: types.StringContent("x")}},
+		})
+		errCh <- err
+	}()
+	<-started
+	cancel() // client disconnects mid-request
+	if err := <-errCh; err == nil {
+		t.Fatal("cancelled request must return an error")
+	}
+	close(release)
+	eventually(t, 2*time.Second, func() bool {
+		_, fc, _ := p.ModelHealthInfo("m")
+		return fc == 0
+	}, "client cancel must not record a scheduling failure")
+}
+
+// TestChatStreamEmptyBodyNotSuccess: a 200 that yields zero chunks and no
+// [DONE] is a failure before the first chunk — the slot must back off.
+func TestChatStreamEmptyBodyNotSuccess(t *testing.T) {
+	up := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "text/event-stream")
+		// 200 with an empty body: no chunks, no [DONE]
+	}))
+	defer up.Close()
+	p := newTestProvider(t, src("mock", up.URL, "openai", "m"))
+	ch, err := p.ChatStream(context.Background(), &types.ChatRequest{
+		Model:    "m",
+		Messages: []types.ChatMessage{{Role: "user", Content: types.StringContent("x")}},
+	})
+	if err != nil {
+		t.Fatalf("stream: %v", err)
+	}
+	got := 0
+	for range ch {
+		got++
+	}
+	if got != 0 {
+		t.Fatalf("want empty stream, got %d chunks", got)
+	}
+	eventually(t, 2*time.Second, func() bool {
+		_, fc, _ := p.ModelHealthInfo("m")
+		return fc >= 1
+	}, "empty stream must record a failure")
+}
+
+// TestImageAutoUsesImageModel: AUTO image generation on a mixed source must
+// send the image-kind model id, never the best chat model.
+func TestImageAutoUsesImageModel(t *testing.T) {
+	var gotModel string
+	up := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		var body map[string]interface{}
+		_ = json.NewDecoder(r.Body).Decode(&body)
+		gotModel, _ = body["model"].(string)
+		fmt.Fprint(w, `{"created":1,"data":[{"url":"http://x/1.png"}]}`)
+	}))
+	defer up.Close()
+	s := config.Source{Name: "mix", BaseURL: up.URL, Adapter: "openai", MaxConcurrent: 4}
+	s.Models = []config.Model{
+		{ID: "chat-m", Kind: "chat", Priority: 100},
+		{ID: "img-m", Kind: "image", Priority: 50},
+	}
+	p := newTestProvider(t, s)
+	resp, err := p.Image(context.Background(), &types.ImageGenRequest{Prompt: "cat", Model: "AUTO"})
+	if err != nil {
+		t.Fatalf("image: %v", err)
+	}
+	if len(resp.ImageData) == 0 {
+		t.Fatal("no image data returned")
+	}
+	if gotModel != "img-m" {
+		t.Fatalf("AUTO image must use the image-kind model, got %q", gotModel)
+	}
+}