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/lua/vm_test.go

@@ -2,6 +2,7 @@ package lua
 
 import (
 	"encoding/json"
+	"os"
 	"path/filepath"
 	"strings"
 	"testing"
@@ -28,13 +29,56 @@ func TestLoadBundledAdapters(t *testing.T) {
 	for _, a := range adapters {
 		names[a.Name] = true
 	}
-	for _, want := range []string{"openai", "deepseek", "anthropic", "gemini", "ollama", "kimicode"} {
+	for _, want := range []string{"openai", "deepseek", "anthropic", "gemini", "ollama", "kimicode", "opencode"} {
 		if !names[want] {
 			t.Errorf("missing adapter %s (got %v)", want, names)
 		}
 	}
 }
 
+func TestFirstRunSeedsEveryBundledAdapter(t *testing.T) {
+	dir := filepath.Join(t.TempDir(), "adapters")
+	vm := NewVM(dir)
+	if err := vm.Start(); err != nil {
+		t.Fatalf("start: %v", err)
+	}
+	defer vm.Stop()
+	// every embedded .lua must have been copied out (and no extra, non-.lua files)
+	embedded, err := bundledAdapters.ReadDir("adapters")
+	if err != nil {
+		t.Fatal(err)
+	}
+	seeded := 0
+	for _, e := range embedded {
+		if e.IsDir() || !strings.HasSuffix(e.Name(), ".lua") {
+			continue
+		}
+		seeded++
+		if _, err := os.Stat(filepath.Join(dir, e.Name())); err != nil {
+			t.Errorf("embedded adapter %s not seeded", e.Name())
+		}
+	}
+	if seeded == 0 {
+		t.Fatal("no embedded adapters found")
+	}
+	written := 0
+	entries, _ := os.ReadDir(dir)
+	for _, e := range entries {
+		if !e.IsDir() && strings.HasSuffix(e.Name(), ".lua") {
+			written++
+		}
+	}
+	if written != seeded {
+		t.Fatalf("seeded %d files but %d embedded adapters exist", written, seeded)
+	}
+	// re-start with the existing dir: authoritative, never rewritten
+	vm2 := NewVM(dir)
+	if err := vm2.Start(); err != nil {
+		t.Fatalf("second start: %v", err)
+	}
+	defer vm2.Stop()
+}
+
 func TestTransformRequest(t *testing.T) {
 	vm := NewVM(freshAdapterDir(t))
 	if err := vm.Start(); err != nil {
@@ -65,6 +109,32 @@ func TestBuildHeadersFallbackStatic(t *testing.T) {
 	}
 }
 
+func TestOpenCodeAdapterFingerprint(t *testing.T) {
+	vm := NewVM(freshAdapterDir(t))
+	if err := vm.Start(); err != nil {
+		t.Fatal(err)
+	}
+	defer vm.Stop()
+	hdrs, err := vm.BuildHeaders("opencode", map[string]interface{}{"api_key": "public"})
+	if err != nil {
+		t.Fatalf("build headers: %v", err)
+	}
+	if ua := hdrs["User-Agent"]; ua != "opencode/0.1.0" {
+		t.Fatalf("opencode adapter must send the opencode client UA, got %q", ua)
+	}
+	if hdrs["Authorization"] != "" {
+		t.Fatalf("opencode adapter must not hardcode Authorization (config api_key supplies it), got %q", hdrs["Authorization"])
+	}
+	// passthrough behaves like the openai adapter
+	out, err := vm.Transform("opencode", "transform_request", `{"model":"x","disable_thinking":true,"extra_body":{},"messages":[{"role":"user","content":"hi"}]}`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if strings.Contains(out, "disable_thinking") || strings.Contains(out, "extra_body") {
+		t.Fatalf("opencode transform_request must strip provider fields: %s", out)
+	}
+}
+
 func TestBuildHeadersCustomHook(t *testing.T) {
 	vm := NewVM(freshAdapterDir(t))
 	if err := vm.Start(); err != nil {