feat(proxy): auto-discovery de modelos + backend Kimchi

- Nueva interfaz Discoverer con Refresh() por backend
- Kilo: full auto (isFree + metadata completa del upstream)
- OpenCode: heurística -free para descubrir nuevos free
- Kimchi: agrega todos los IDs no-curados del upstream
- discoveryLoop cada 10 min en el Proxy
- Backends stateful (mutex + catálogo dinámico)
- CREDENCIALES.md y KIMCHI_ANALISIS.md documentan el setup

internal/proxy/backend.go

@@ -11,6 +11,8 @@ type Backend interface {
 	Name() string
 
 	// Models returns the free models this backend exposes, for /v1/models listing.
+	// Implementations may return a dynamic (discovered) catalogue; before the
+	// first successful Refresh() they fall back to the curated list.
 	Models() []ModelInfo
 
 	// Resolve maps a client-requested model name to the upstream model ID.
@@ -25,3 +27,16 @@ type Backend interface {
 	// request to the upstream (auth, client-id, request/session tracking, etc.).
 	Headers(requestID, sessionID string) map[string]string
 }
+
+// Discoverer is an optional capability a Backend may implement to
+// auto-discover its model catalogue from the upstream /models endpoint.
+//
+// The Proxy runs a discovery loop that calls Refresh() periodically. Each
+// implementation owns its parsing and free-model filtering logic, and merges
+// discovered models with its curated base layer (curated metadata always wins
+// for known IDs).
+type Discoverer interface {
+	// Refresh fetches the upstream model list and updates the backend's
+	// dynamic catalogue. Safe to call concurrently with Models()/Resolve().
+	Refresh() error
+}