Fix RuboCop lint failures in examples and chat client

[Copilot]

The CI lint job was failing with 3 autocorrectable RuboCop offenses.

Changes

• examples/unify_api.rb: Remove useless assignments to chat and git — these objects were constructed but never referenced; the script already uses unify.chat / unify.git via the client.unify facade
• lib/bundleup/unify/chat.rb: Flatten multi-line unless block in #message to modifier form, consistent with the rest of the file

# before
unless response.success?
  raise "Failed to post chat/channels/#{encoded_channel_id}/message: #{response.status}"
end

# after
raise "Failed to post chat/channels/#{encoded_channel_id}/message: #{response.status}" unless response.success?