Consistently use double quotes in ApplicationAgent template

[marcoroth]

Running rails generate active_agent:install generates a app/agents/application_agent.rb file using mixed double and single quotes:

class ApplicationAgent < ActiveAgent::Base
  layout 'agent'
  
  generate_with :openai, model: "gpt-4o-mini", instructions: "You are a helpful assistant."
end

This pull request updates the template to use double quotes for all strings in that file. Additionally, it removes the duplicate/unused ApplicationAgent template file in lib/generators/erb/templates/application_agent.rb.tt.

[TonsOfFun]

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