fix quickstart: ensure original_signature is Signature

[dimroc]

Currently, the quickstart example errors because the CoT original_signature remains a string rather than a Signature instance.

 result = cot.call(question: "Two dice are tossed. What is the probability that the sum equals two?")
[Desiru] 2025-06-26 11:07:40 -0700: WARN -- Retrying module execution (attempt 1/3)
[Desiru] 2025-06-26 11:07:41 -0700: WARN -- Retrying module execution (attempt 2/3)
[Desiru] 2025-06-26 11:07:42 -0700: WARN -- Retrying module execution (attempt 3/3)
[Desiru] 2025-06-26 11:07:43 -0700: ERROR -- Module execution failed: undefined method 'output_fields' for an instance of String
(ponds):2:in '<main>': Module execution failed: undefined method 'output_fields' for an instance of String (Desiru::ModuleError)
/Users/dimroc/.gem/ruby/3.4.2/bundler/gems/desiru-b5b302af0e3b/lib/desiru/modules/chain_of_thought.rb:26:in 'Desiru::Modules::ChainOfThought#build_system_prompt': undefined method 'output_fields' for an instance of String (NoMethodError)

          #{@original_signature.output_fields.keys.map { |field| "#{field}: Your #{field} here" }.join("\n")}
                               ^^^^^^^^^^^^^^
        from /Users/dimroc/.gem/ruby/3.4.2/bundler/gems/desiru-b5b302af0e3b/lib/desiru/modules/predict.rb:33:in 'Desiru::Modules::Predict#build_prompt'
        from /Users/dimroc/.gem/ruby/3.4.2/bundler/gems/desiru-b5b302af0e3b/lib/desiru/modules/predict.rb:15:in 'Desiru::Modules::Predict#forward'
        from /Users/dimroc/.gem/ruby/3.4.2/bundler/gems/desiru-b5b302af0e3b/lib/desiru/module.rb:50:in 'Desiru::Module#call'
        from /Users/dimroc/.gem/ruby/3.4.2/bundler/gems/desiru-b5b302af0e3b/lib/desiru/core/traceable.rb:20:in 'Desiru::Core::Traceable#call'

This PR ensures the ChainOfThought#original_signature becomes a Signature instance. To implement this, there were a few liberties taken:

1. DRY'd up the signature wrapping into Signature.wrap
2. Made ChainOfThought#original_signature a public attr_reader for easier access and testability, mimicing Module#signature.

Testing details and red, green refactor

Original error: