Add minimal example client flow

[TheNorthStarYang]

Goal

Add or improve a minimal example client flow so developers can understand how to start talking to an AhaKey device.

Scope

• focus on a minimal and understandable flow
• avoid overcomplicating examples
• avoid unrelated refactors

Where to edit

• repository: protocol
• relevant path: examples/minimal-flow.md
• related files: examples/README.md

Notes

This issue is part of the early public protocol onboarding work.

Contributors are welcome.