Expand the agent provider structure and add some configs for the claude provider

I've separated the structure of the claude agent implementation into three main pieces:

1. the `agent` cog -- generic for all agent providers
2. the `claude` provider -- implements a generic interface; can be instantiated and invoked by the agent
3. `ClaudeInvocation` -- an internal implementation detail of the Claude provider that wraps a single run of the Claude Code CLI (it will be helpful to have a stateful container unique to a specific invocation for output formatting, session tracking, etc.)

Author: juniper-shopify

dsl/simple_agent.rb

@@ -6,9 +6,12 @@
 config do
   agent do
     provider :claude
+    model "haiku"
+    initial_prompt "Always respond in haiku form"
+    show_prompt!
   end
 end
 
 execute do
-  agent(:foo) { "Say hi" }
+  agent { "What is the world's largest lake?" }
 end