Deprecate preserve: parameter in annotate_file

Rewriter always preserves formatting, so preserve: is no longer needed.
Default it to true and mark the overload with preserve: as deprecated.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: soutaro

lib/rbs/annotate/rdoc_annotator.rb

@@ -13,7 +13,7 @@ def initialize(source:)
         @include_filename = true
       end
 
-      def annotate_file(path, preserve:)
+      def annotate_file(path, preserve: true)
         buffer, _, decls = Parser.parse_signature(path.read())
 
         rewriter = Rewriter.new(buffer)

sig/annotate/rdoc_annotater.rbs

@@ -8,7 +8,8 @@ module RBS
 
       def initialize: (source: RDocSource) -> void
 
-      def annotate_file: (Pathname, preserve: bool) -> void
+      def annotate_file: (Pathname) -> void
+                       | %a{deprecated: it preserves the format by default} (Pathname, preserve: bool) -> void
 
       def annotate_decls: (Array[AST::Declarations::t], Rewriter, ?outer: Array[Namespace]) -> void