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/config/config.go

@@ -6,6 +6,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"os"
+	"path/filepath"
 	"time"
 
 	"gopkg.in/yaml.v3"
@@ -63,8 +64,13 @@ type Source struct {
 	QueueTimeout  time.Duration          `yaml:"queue_timeout" json:"-"`
 }
 
-// Load reads and validates a config file.
+// Load reads and validates a config file. When the file does not exist yet a
+// default config is generated at that path first (first-run bootstrap), so a
+// fresh binary just works: `llmsproxy -config /path/to/config.yaml`.
 func Load(path string) (*Config, error) {
+	if _, err := EnsureDefault(path); err != nil {
+		return nil, err
+	}
 	data, err := os.ReadFile(path)
 	if err != nil {
 		return nil, err
@@ -80,6 +86,63 @@ func Load(path string) (*Config, error) {
 	return &cfg, nil
 }
 
+// EnsureDefault creates a default config file at path when it does not exist
+// yet and returns whether it was created. The repo ships no config file
+// (config files carry real keys); the binary generates one per install with a
+// fresh random admin key. An existing file is never touched.
+func EnsureDefault(path string) (bool, error) {
+	if _, err := os.Stat(path); err == nil {
+		return false, nil
+	} else if !os.IsNotExist(err) {
+		return false, err
+	}
+	if err := writeDefaultConfig(path); err != nil {
+		return false, err
+	}
+	return true, nil
+}
+
+// writeDefaultConfig writes a minimal, safe-by-default config: loopback-only
+// listen, a fresh random admin key, and a no-key zen source that works out of
+// the box. adapter_dir / runtime_file live next to the config file so the
+// binary works regardless of the working directory it is started from.
+func writeDefaultConfig(path string) error {
+	key, err := NewGatewayKey()
+	if err != nil {
+		return fmt.Errorf("generate gateway key: %w", err)
+	}
+	dir := filepath.Dir(path)
+	abs, err := filepath.Abs(dir)
+	if err != nil {
+		abs = dir
+	}
+	cfg := Config{
+		Listen:       "127.0.0.1:8080",
+		GatewayKeys:  []string{key},
+		DefaultModel: "AUTO",
+		AdapterDir:   filepath.Join(abs, "adapters"),
+		RuntimeFile:  filepath.Join(abs, "runtime.json"),
+		Sources: []Source{
+			{
+				Name:    "zen",
+				BaseURL: "https://opencode.ai/zen/v1",
+				APIKey:  "public", // zen 免费池：官方无 key 客户端实际发送 Bearer public
+				Adapter: "opencode",
+				Models:  []Model{{ID: "deepseek-v4-flash-free", Priority: 100, Kind: "chat"}},
+			},
+		},
+	}
+	out, err := yaml.Marshal(&cfg)
+	if err != nil {
+		return fmt.Errorf("marshal default config: %w", err)
+	}
+	// The config holds the plaintext admin key — restrict permissions.
+	if err := os.MkdirAll(abs, 0755); err != nil {
+		return fmt.Errorf("mkdir config dir: %w", err)
+	}
+	return os.WriteFile(path, out, 0600)
+}
+
 // RemoveSourceFromYAML deletes the named source entry from the config file so
 // the delete is a real one (no tombstone needed). Uses yaml.Node to preserve
 // the rest of the file's comments and formatting.