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.go

@@ -302,13 +302,21 @@ func (v *VM) pool(name string) *adapterPool {
 }
 
 func (v *VM) writeBundledAdapters() error {
-	known := []string{"openai", "anthropic", "deepseek", "gemini", "github", "groq", "mistral", "ollama", "kimicode"}
-	for _, name := range known {
-		dst := filepath.Join(v.dir, name+".lua")
+	// Derive the set from the embedded FS instead of a hardcoded name list so
+	// adding an adapter never requires maintaining a second list.
+	entries, err := bundledAdapters.ReadDir("adapters")
+	if err != nil {
+		return err
+	}
+	for _, e := range entries {
+		if e.IsDir() || !strings.HasSuffix(e.Name(), ".lua") {
+			continue
+		}
+		dst := filepath.Join(v.dir, e.Name())
 		if _, err := os.Stat(dst); err == nil {
 			continue
 		}
-		data, err := bundledAdapters.ReadFile("adapters/" + name + ".lua")
+		data, err := bundledAdapters.ReadFile("adapters/" + e.Name())
 		if err != nil {
 			continue
 		}