fix(config): avoid writing known defaults during merge

internal/config/config.go

@@ -1098,8 +1098,13 @@ func getOrCreateMapValue(mapNode *yaml.Node, key string) *yaml.Node {
 
 // mergeMappingPreserve merges keys from src into dst mapping node while preserving
 // key order and comments of existing keys in dst. New keys are only added if their
-// value is non-zero to avoid polluting the config with defaults.
-func mergeMappingPreserve(dst, src *yaml.Node) {
+// value is non-zero and not a known default to avoid polluting the config with defaults.
+func mergeMappingPreserve(dst, src *yaml.Node, path ...[]string) {
+	var currentPath []string
+	if len(path) > 0 {
+		currentPath = path[0]
+	}
+
 	if dst == nil || src == nil {
 		return
 	}
@@ -1113,13 +1118,14 @@ func mergeMappingPreserve(dst, src *yaml.Node) {
 		sk := src.Content[i]
 		sv := src.Content[i+1]
 		idx := findMapKeyIndex(dst, sk.Value)
+		childPath := appendPath(currentPath, sk.Value)
 		if idx >= 0 {
 			// Merge into existing value node (always update, even to zero values)
 			dv := dst.Content[idx+1]
-			mergeNodePreserve(dv, sv)
+			mergeNodePreserve(dv, sv, childPath)
 		} else {
-			// New key: only add if value is non-zero to avoid polluting config with defaults
-			if isZeroValueNode(sv) {
+			// New key: only add if value is non-zero and not a known default
+			if isKnownDefaultValue(childPath, sv) {
 				continue
 			}
 			dst.Content = append(dst.Content, deepCopyNode(sk), deepCopyNode(sv))
@@ -1130,7 +1136,12 @@ func mergeMappingPreserve(dst, src *yaml.Node) {
 // mergeNodePreserve merges src into dst for scalars, mappings and sequences while
 // reusing destination nodes to keep comments and anchors. For sequences, it updates
 // in-place by index.
-func mergeNodePreserve(dst, src *yaml.Node) {
+func mergeNodePreserve(dst, src *yaml.Node, path ...[]string) {
+	var currentPath []string
+	if len(path) > 0 {
+		currentPath = path[0]
+	}
+
 	if dst == nil || src == nil {
 		return
 	}
@@ -1139,7 +1150,7 @@ func mergeNodePreserve(dst, src *yaml.Node) {
 		if dst.Kind != yaml.MappingNode {
 			copyNodeShallow(dst, src)
 		}
-		mergeMappingPreserve(dst, src)
+		mergeMappingPreserve(dst, src, currentPath)
 	case yaml.SequenceNode:
 		// Preserve explicit null style if dst was null and src is empty sequence
 		if dst.Kind == yaml.ScalarNode && dst.Tag == "!!null" && len(src.Content) == 0 {
@@ -1162,7 +1173,7 @@ func mergeNodePreserve(dst, src *yaml.Node) {
 				dst.Content[i] = deepCopyNode(src.Content[i])
 				continue
 			}
-			mergeNodePreserve(dst.Content[i], src.Content[i])
+			mergeNodePreserve(dst.Content[i], src.Content[i], currentPath)
 			if dst.Content[i] != nil && src.Content[i] != nil &&
 				dst.Content[i].Kind == yaml.MappingNode && src.Content[i].Kind == yaml.MappingNode {
 				pruneMissingMapKeys(dst.Content[i], src.Content[i])
@@ -1204,6 +1215,56 @@ func findMapKeyIndex(mapNode *yaml.Node, key string) int {
 	return -1
 }
 
+// appendPath appends a key to the path, returning a new slice to avoid modifying the original.
+func appendPath(path []string, key string) []string {
+	if len(path) == 0 {
+		return []string{key}
+	}
+	newPath := make([]string, len(path)+1)
+	copy(newPath, path)
+	newPath[len(path)] = key
+	return newPath
+}
+
+// isKnownDefaultValue returns true if the given node at the specified path
+// represents a known default value that should not be written to the config file.
+// This prevents non-zero defaults from polluting the config.
+func isKnownDefaultValue(path []string, node *yaml.Node) bool {
+	// First check if it's a zero value
+	if isZeroValueNode(node) {
+		return true
+	}
+
+	// Match known non-zero defaults by exact dotted path.
+	if len(path) == 0 {
+		return false
+	}
+
+	fullPath := strings.Join(path, ".")
+
+	// Check string defaults
+	if node.Kind == yaml.ScalarNode && node.Tag == "!!str" {
+		switch fullPath {
+		case "pprof.addr":
+			return node.Value == DefaultPprofAddr
+		case "remote-management.panel-github-repository":
+			return node.Value == DefaultPanelGitHubRepository
+		case "routing.strategy":
+			return node.Value == "round-robin"
+		}
+	}
+
+	// Check integer defaults
+	if node.Kind == yaml.ScalarNode && node.Tag == "!!int" {
+		switch fullPath {
+		case "error-logs-max-files":
+			return node.Value == "10"
+		}
+	}
+
+	return false
+}
+
 // isZeroValueNode returns true if the YAML node represents a zero/default value
 // that should not be written as a new key to preserve config cleanliness.
 // For mappings and sequences, recursively checks if all children are zero values.