feat: Add custom_label_4 handling in API and update documentation for new endpoints

.serena/project.yml

@@ -127,3 +127,8 @@ language_backend:
 # list of regex patterns which, when matched, mark a memory entry as read‑only.
 # Extends the list from the global configuration, merging the two lists.
 read_only_memory_patterns: []
+
+# line ending convention to use when writing source files.
+# Possible values: unset (use global setting), "lf", "crlf", or "native" (platform default)
+# This does not affect Serena's own files (e.g. memories and configuration files), which always use native line endings.
+line_ending: