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

cmd/llmsproxy/main.go

@@ -16,6 +16,7 @@ import (
 	"syscall"
 	"time"
 
+	"llmsproxy/internal/config"
 	"llmsproxy/internal/core"
 	"llmsproxy/internal/gateway"
 )
@@ -24,12 +25,23 @@ func main() {
 	cfgPath := flag.String("config", "config.yaml", "path to gateway config file")
 	flag.Parse()
 
+	created, err := config.EnsureDefault(*cfgPath)
+	if err != nil {
+		log.Fatalf("[llmsproxy] config: %v", err)
+	}
+
 	c, err := core.New(*cfgPath)
 	if err != nil {
 		log.Fatalf("[llmsproxy] core: %v", err)
 	}
 	defer c.Close()
 
+	if created {
+		for _, k := range c.GatewayKeys() {
+			log.Printf("[llmsproxy] generated default config at %s — admin key: %s (rotate it in the WebUI after first login)", *cfgPath, k)
+		}
+	}
+
 	gw, err := gateway.New(c, c.GatewayKeys())
 	if err != nil {
 		log.Fatalf("[llmsproxy] gateway: %v", err)