feat(watcher): add Gemini models and OAuth model mappings change detection

internal/watcher/diff/oauth_excluded.go

@@ -122,6 +122,11 @@ type VertexModelsSummary struct {
 	count int
 }
 
+type GeminiModelsSummary struct {
+	hash  string
+	count int
+}
+
 // SummarizeVertexModels hashes vertex-compatible models for change detection.
 func SummarizeVertexModels(models []config.VertexCompatModel) VertexModelsSummary {
 	if len(models) == 0 {
@@ -149,3 +154,24 @@ func SummarizeVertexModels(models []config.VertexCompatModel) VertexModelsSummar
 		count: len(names),
 	}
 }
+
+// SummarizeGeminiModels hashes Gemini model aliases for change detection.
+func SummarizeGeminiModels(models []config.GeminiModel) GeminiModelsSummary {
+	if len(models) == 0 {
+		return GeminiModelsSummary{}
+	}
+	keys := normalizeModelPairs(func(out func(key string)) {
+		for _, model := range models {
+			name := strings.TrimSpace(model.Name)
+			alias := strings.TrimSpace(model.Alias)
+			if name == "" && alias == "" {
+				continue
+			}
+			out(strings.ToLower(name) + "|" + strings.ToLower(alias))
+		}
+	})
+	return GeminiModelsSummary{
+		hash:  hashJoined(keys),
+		count: len(keys),
+	}
+}