docs: outline agenda action descriptors

[marinasundstrom]

Summary

• expand the agenda item design note with action descriptors and meeting-level coordination guidance
• document the standard handler contract and how it returns the next set of available actions
• describe the action graph that powers previews and follow-up recommendations after each step

Testing

• not run (documentation-only change)

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https://chatgpt.com/codex/tasks/task_e_68fcb6944418832fa8f9795e4af88968