feat: add multi-key support with automatic rate-limit rotation

Support multiple API keys via `api_keys` array in config or `OCGO_API_KEYS`
env var. On HTTP 429, automatically rotate to the next key and retry.
Key index persisted to `~/.config/ocgo/key-index` for restart safety.
Backward compatible with existing single-key `api_key` configs.

Fixes #115

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

cmd/ocgo/main.go

@@ -197,6 +197,16 @@ func knownModelIDs() []string {
 	return []string{"glm-5.1", "glm-5", "kimi-k2.6", "kimi-k2.5", "mimo-v2.5-pro", "mimo-v2.5", "mimo-v2-pro", "mimo-v2-omni", "minimax-m2.7", "minimax-m2.5", "deepseek-v4-pro[1M]", "deepseek-v4-flash[1M]", "qwen3.6-plus", "qwen3.5-plus"}
 }
 
+func isKnownModelID(model string) bool {
+	cleaned := cleanModelName(model)
+	for _, m := range knownModelIDs() {
+		if cleanModelName(m) == cleaned {
+			return true
+		}
+	}
+	return false
+}
+
 func cleanModelName(model string) string {
 	model = strings.TrimSuffix(model, "[1M]")
 	model = strings.TrimSuffix(model, "[128K]")
@@ -450,7 +460,7 @@ func proxyChatCompletions(w http.ResponseWriter, r *http.Request, cfg Config) {
 		dm := cleanModelName(cfg.Model)
 		var reqMap map[string]any
 		if json.Unmarshal(body, &reqMap) == nil {
-			if m, _ := reqMap["model"].(string); m == "" || strings.HasPrefix(m, "claude-") {
+			if m, _ := reqMap["model"].(string); m == "" || !isKnownModelID(m) {
 				reqMap["model"] = dm
 				body, _ = json.Marshal(reqMap)
 			}
@@ -615,7 +625,7 @@ func stripRawChatImageDetails(req map[string]any) bool {
 
 func convertRequest(ar AnthropicRequest, defaultModel string) OAIRequest {
 	model := cleanModelName(ar.Model)
-	if model == "" || strings.HasPrefix(model, "claude-") || defaultModel != "" {
+	if model == "" || !isKnownModelID(model) {
 		model = cleanModelName(defaultModel)
 	}
 	if model == "" {
@@ -636,7 +646,7 @@ func convertRequest(ar AnthropicRequest, defaultModel string) OAIRequest {
 
 func responsesToChat(rr ResponsesRequest, defaultModel string) OAIRequest {
 	model := cleanModelName(rr.Model)
-	if model == "" || defaultModel != "" {
+	if model == "" || !isKnownModelID(model) {
 		model = cleanModelName(defaultModel)
 	}
 	if model == "" {