docs: add ai_agent feedback destination for ext-feedback

[maximelb]

Summary

• Add ai_agent as a new feedback destination in the ext-feedback docs (alongside case and playbook)
• Document the ai_agent_name parameter across all three feedback actions (request_simple_approval, request_acknowledgement, request_question)
• Add "AI agent follow-up" to the human-in-the-loop tutorial's "Extending the Pattern" section

Test plan

• Verify rendered tables in feedback.md show the new ai_agent destination row and ai_agent_name parameter rows
• Verify the tutorial renders the new paragraph correctly

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