chore: fix 14 trivial RuboCop cops

[mroderick]

What

Fixes 14 RuboCop cops (safe autocorrect only), one commit per rule, then regenerates .rubocop_todo.yml.

Cops fixed

Cop	Offenses	Files
Lint/AmbiguousOperator	1	workshops_controller_spec.rb
Lint/RedundantStringCoercion	1	admin/sponsor_spec.rb
RSpec/BeEmpty	1	event_presenter_spec.rb
RSpec/ChangeByZero	3	admin/sponsors_controller_spec.rb
RSpec/ItBehavesLike	2	workshop_spec.rb
Rails/FilePath	1	admin/sponsors_controller_spec.rb
Style/CommentAnnotation	1	invitation_manager_spec.rb
Style/DefWithParentheses	4	email_header_helper.rb
Style/ExpandPathArguments	1	spec_helper.rb
Style/HashAsLastArrayItem	1	sponsor.rb
Style/Lambda	1	workshop_invitation.rb
Style/NegatedIf	2	setup.rake
Style/NumericLiteralPrefix	1	admin/workshops_spec.rb
Style/RedundantReturn	1	omniauth/codebar.rb
Style/RegexpLiteral	1	spec_helper.rb

15 cops total (2 in 1 shared commit where needed to pass pre-commit hook). RSpec/ImplicitSubject deliberately skipped — its autocorrect triggers RSpec/NamedSubject which is a bigger refactor.

22 offenses fixed across 15 files, all safe autocorrect.