Add RuboCop

[d-mato]

Adds RuboCop and brings the tree in line with it. The config is kept to what this repository actually needs.

# TargetRubyVersion is left unset on purpose: RuboCop reads it from
# required_ruby_version in the gemspec, which ranks above .ruby-version.
AllCops:
  NewCops: enable

Style/StringLiterals:
  EnforcedStyle: double_quotes

Metrics/BlockLength:
  Exclude:
    - spec/**/*

Three deliberate departures from what bundle gem generates:

• No TargetRubyVersion. RuboCop::TargetRuby::SOURCES orders GemspecFile above RubyVersionFile, so required_ruby_version in the gemspec wins over .ruby-version. rubocop -V confirms it: analyzing as Ruby 3.3, not 4.0.6. Writing the version here would duplicate the gemspec and quietly drift from it.
• No Style/StringLiteralsInInterpolation. There is no interpolated string literal in the tree for it to govern.
• NewCops: enable. Without it every run prints a growing list of pending cops. With it the config never needs editing to stay current; a RuboCop bump that introduces failures shows up in its own Dependabot PR.

Style/StringLiterals is set to double quotes because the tree is already 72 double to 19 single. RuboCop defaults to single, which would have rewritten the gemspec added in #21.

RuboCop got one wrong

Security/JSONLoad is an unsafe autocorrection and it broke the gem:

JSON.parse(File.open(File.join(__dir__, "data", "emoji_ja.json")))
# => TypeError: no implicit conversion of File into String

JSON.load accepts an IO, JSON.parse does not. The dictionary is read with File.read instead, which also closes the handle File.open was leaking. Every public method raised TypeError before this was corrected.

Not addressed here

Style/TrailingCommaInArrayLiteral removed the trailing comma in tasks/data.rake, leaving the commented-out entries below it comma-fragile — uncommenting one now yields a syntax error. Those entries have been commented out since 2020; deleting them is probably better than configuring around them, but that is a separate call.

Verification

rubocop: 9 files inspected, no offenses. Suite green locally (11 examples) and gem bump is unaffected — gem-release matches VERSION\s*=\s*("|) anywhere in the file, so the added magic comment does not interfere.