Apply PR review feedback: frozen string literals and file handle fix

[Copilot]

Why?

Three issues identified in code review of the rolemodel-rails plugin rework:

• Missing # frozen_string_literal: true in three new/modified files
• File.open in clean_test_gemfile left a file descriptor unclosed across repeated generator specs

What Changed

• Add # frozen_string_literal: true to lib/rolemodel-rails.rb, lib/generators/rolemodel/base_generator.rb, and spec/spec_helper.rb
• Replace File.open with File.readlines in spec/support/helpers/example_app.rb#clean_test_gemfile to close the descriptor deterministically

# before
gemfile = File.open(gemfile_path)
File.write(gemfile_path, gemfile.grep_v(/rolemodel-rails/).join)

# after
File.write(gemfile_path, File.readlines(gemfile_path).grep_v(/rolemodel-rails/).join)

Pre-merge checklist

• Update relevant READMEs
• Run bin/bump_version or bin/bump_version --patch

Screenshots

N/A

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