Implement Pathname#rmtree in builtin Pathname

Move rmtree from lib/pathname.rb (which delegated to FileUtils.rm_rf)
into pathname_builtin.rb with a pure Ruby implementation using
File.lstat, Dir.each_child, Dir.rmdir, and File.unlink.

The noop, verbose, and secure keyword arguments are intentionally
dropped to remove the fileutils dependency.

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

Author: hsbt

lib/pathname.rb

@@ -10,21 +10,6 @@
 # For documentation, see class Pathname.
 #
 
-class Pathname    # * FileUtils *
-  # Recursively deletes a directory, including all directories beneath it.
-  #
-  # Note that you need to require 'pathname' to use this method.
-  #
-  # See FileUtils.rm_rf
-  def rmtree(noop: nil, verbose: nil, secure: nil)
-    # The name "rmtree" is borrowed from File::Path of Perl.
-    # File::Path provides "mkpath" and "rmtree".
-    require 'fileutils'
-    FileUtils.rm_rf(@path, noop: noop, verbose: verbose, secure: secure)
-    self
-  end
-end
-
 class Pathname    # * tmpdir *
   # call-seq:
   #   Pathname.mktmpdir -> new_pathname

pathname_builtin.rb

@@ -1383,6 +1383,36 @@ def unlink()
     File.unlink @path
   end
   alias delete unlink
+
+  # Recursively deletes a directory, including all directories beneath it.
+  #
+  # The name "rmtree" is borrowed from File::Path of Perl.
+  # File::Path provides "mkpath" and "rmtree".
+  #
+  # The previous implementation delegated to FileUtils.rm_rf and accepted
+  # +noop+, +verbose+, and +secure+ keyword arguments. This builtin
+  # implementation intentionally drops them to remove the fileutils dependency.
+  #
+  #   Pathname("/tmp/testdir").rmtree
+  #
+  def rmtree
+    remove_entry(@path)
+    self
+  end
+
+  private
+
+  def remove_entry(path)
+    st = File.lstat(path)
+    if st.directory?
+      Dir.each_child(path) do |child|
+        remove_entry(File.join(path, child))
+      end
+      Dir.rmdir(path)
+    else
+      File.unlink(path)
+    end
+  end
 end
 
 class Pathname