feat: OpenAI, Anthropic, and native-Ollama providers + media pipeline

Phase 3:
- provider/openai: Chat Completions for OpenAI + compat endpoints (SSE
  streaming with by-index tool-call assembly, response_format json_schema,
  legacy max_tokens option, reasoning_effort)
- provider/anthropic: Messages API (tool_use/tool_result, GA structured
  output via output_config.format, full SSE event parser, 529 transient)
- provider/ollama: one native /api/chat client behind the ollama,
  ollama-cloud, and foreman built-ins (presets; NDJSON streaming tolerant
  of foreman's buffered single-object responses; object tool arguments;
  format-schema structured output; think mapping)
- media/: capability normalization (sniff, downscale, transcode, byte
  ladder, ErrUnsupported), wired into the chain executor per target with
  penalty-free advance past incapable elements
- registry: real provider + scheme wiring, WithHTTPClient option, required
  env-foreman TLS chat round-trip test
- ADR-0009 multimodal strategy, ADR-0010 tools/structured mapping; README
  matrix + CLAUDE.md synced

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

failover_test.go

@@ -1,8 +1,12 @@
 package majordomo
 
 import (
+	"bytes"
 	"context"
 	"errors"
+	"image"
+	"image/color"
+	"image/png"
 	"slices"
 	"strings"
 	"sync"
@@ -325,6 +329,95 @@ func TestSingleTargetGetsChainSemantics(t *testing.T) {
 	}
 }
 
+// pngImage encodes a width×height PNG for media tests.
+func pngImage(t *testing.T, width, height int) []byte {
+	t.Helper()
+	img := image.NewRGBA(image.Rect(0, 0, width, height))
+	for y := range height {
+		for x := range width {
+			img.Set(x, y, color.RGBA{R: uint8(x), G: uint8(y), B: 128, A: 255})
+		}
+	}
+	var buf bytes.Buffer
+	if err := png.Encode(&buf, img); err != nil {
+		t.Fatalf("encode png: %v", err)
+	}
+	return buf.Bytes()
+}
+
+// TestChainNormalizesMediaPerTarget: the request's image is downscaled to
+// the capabilities of the target that actually serves it.
+func TestChainNormalizesMediaPerTarget(t *testing.T) {
+	r := newTestRegistry(t)
+	fp := fake.New("fp",
+		fake.WithModelCapabilities("small-vision", llm.Capabilities{
+			MaxImagesPerReq:   2,
+			MaxImageDimension: 32,
+			AllowedImageMIME:  []string{"image/png"},
+		}),
+	)
+	r.RegisterProvider(fp)
+
+	m, _ := r.Parse("fp/small-vision")
+	_, err := m.Generate(context.Background(), Request{Messages: []Message{
+		UserParts(Text("describe"), Image("image/png", pngImage(t, 100, 50))),
+	}})
+	if err != nil {
+		t.Fatalf("Generate: %v", err)
+	}
+
+	calls := fp.Calls()
+	if len(calls) != 1 {
+		t.Fatalf("calls = %d", len(calls))
+	}
+	var img llm.ImagePart
+	for _, part := range calls[0].Request.Messages[0].Parts {
+		if ip, ok := part.(llm.ImagePart); ok {
+			img = ip
+		}
+	}
+	if img.Data == nil {
+		t.Fatal("no image reached the provider")
+	}
+	cfg, err := png.DecodeConfig(bytes.NewReader(img.Data))
+	if err != nil {
+		t.Fatalf("decode delivered image: %v", err)
+	}
+	if cfg.Width != 32 || cfg.Height != 16 {
+		t.Errorf("delivered image = %dx%d, want 32x16 (downscaled to target cap)", cfg.Width, cfg.Height)
+	}
+}
+
+// TestChainAdvancesPastImagelessTarget: a text-only head can't take an
+// image request; the chain advances to a vision-capable element with no
+// health penalty.
+func TestChainAdvancesPastImagelessTarget(t *testing.T) {
+	r := newTestRegistry(t)
+	fp := fake.New("fp",
+		fake.WithModelCapabilities("text-only", llm.Capabilities{SupportsTools: true}),
+		fake.WithModelCapabilities("vision", llm.Capabilities{MaxImagesPerReq: 4}),
+	)
+	r.RegisterProvider(fp)
+	fp.Enqueue("vision", fake.Reply("a tasteful png"))
+
+	m, _ := r.Parse("fp/text-only,fp/vision")
+	resp, err := m.Generate(context.Background(), Request{Messages: []Message{
+		UserParts(Text("what is this?"), Image("image/png", pngImage(t, 8, 8))),
+	}})
+	if err != nil {
+		t.Fatalf("Generate: %v", err)
+	}
+	if resp.Text() != "a tasteful png" {
+		t.Errorf("text = %q", resp.Text())
+	}
+	if got := fp.CallCount("text-only"); got != 0 {
+		t.Errorf("text-only target saw %d calls, want 0 (normalization rejects pre-send)", got)
+	}
+	if !r.Health().Available("fp/text-only") {
+		t.Error("media rejection must not penalize health")
+	}
+}
+
 // TestHTTP529ClassifiedTransient: Anthropic's "overloaded" status fails
 // over like any other transient error.
 func TestHTTP529FailsOver(t *testing.T) {