fix(11-04): keep earlier overrides when compass Persist saves

Persist rewrote overrides.yaml with only this process's runtime values,
so saving one key (for example websockets:generate-vapid-keys --update)
dropped every key persisted earlier. It now starts from the saved file
and lets runtime values win.

modules/compass/persist.go

@@ -27,7 +27,8 @@ func (c *Config) Set(path string, value any) error {
 }
 
 // Persist writes runtime overrides to config/env/<env>/overrides.yaml
-// using an atomic replace and restrictive file permissions.
+// using an atomic replace and restrictive file permissions. Keys already in
+// the file are kept; a runtime value replaces the saved value of its key.
 func (c *Config) Persist() error {
 	if c == nil {
 		return fmt.Errorf("compass: config is nil")
@@ -48,9 +49,20 @@ func (c *Config) Persist() error {
 	if err := os.MkdirAll(destDir, 0o700); err != nil {
 		return fmt.Errorf("compass: create overrides dir: %w", err)
 	}
-	raw := []byte("{}\n")
+	// Start from the values persisted earlier, so saving one runtime value
+	// never drops another key from the file; runtime values win.
+	merged := koanf.New(".")
+	if err := loadOverrides(merged, dest); err != nil {
+		return err
+	}
 	if c.runtime != nil {
-		body, err := c.runtime.Marshal(yaml.Parser())
+		if err := merged.Merge(c.runtime); err != nil {
+			return fmt.Errorf("compass: merge overrides: %w", err)
+		}
+	}
+	raw := []byte("{}\n")
+	if len(merged.Keys()) > 0 {
+		body, err := merged.Marshal(yaml.Parser())
 		if err != nil {
 			return fmt.Errorf("compass: marshal overrides: %w", err)
 		}