fix: target Ruby 3.3 instead of 4.0

- Remove nkf gem (only needed for Ruby 4.0, in stdlib for 3.3)
- Add require 'date' for permitted_classes in YAML loading
- Keep Re:VIEW 5.11.0 (needed for Ruby 3.1+ Psych compatibility)
- Add .ruby-version for rbenv (3.3.10)

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

Author: piti6

.ruby-version

@@ -0,0 +1 @@
+3.3.10

Gemfile

@@ -4,5 +4,4 @@ source "https://rubygems.org"
 gem 'review', '5.11.0'
 gem 'pandoc2review'
 gem 'rake'
-gem 'nkf'
 # gem 'review-peg', '0.2.2'

articles/lib/tasks/z01_pandoc2review.rake

@@ -20,6 +20,7 @@
 
 require 'fileutils'
 require 'yaml'
+require 'date'
 
 def make_mdre(ch, p2r, path)
   if File.exist?(ch) # re file