feat: AUTO chain rewrite — silent failover+busy skip+pref round-robin+503 tier summary; chain edits reset slot cooldowns (P0/P1); stats by_status + audit jsonl rotation; UI priority-page health badges & status-code card; ctx-menu capture-phase close (outside-press guard); main.go ops warnings; local bundled-Lua verified tests (3 latent bugs fixed); plan.md

internal/types/types.go

@@ -4,9 +4,17 @@ package types
 
 import (
 	"encoding/json"
+	"errors"
 	"time"
 )
 
+// ErrBusy is the soft "source at capacity" sentinel shared by the provider
+// layer (returns it) and the scheduler layer (reacts to it): busy is not a
+// failure, so no cooldown/preference penalty is recorded, and gateways map
+// it to HTTP 429. Defined here so the scheduler does not depend on the
+// provider package (which pulls in the Lua runtime).
+var ErrBusy = errors.New("provider busy")
+
 // ---- OpenAI wire request (gateway input) ----
 
 type ChatRequest struct {