[WIP] Add steps to install ffmpeg in actions

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

Original prompt

Generate an agentic workflow for ffmpeg:

• provides steps to install in actions (there is probably a custom action)
• prompt to help the agent find ffmpeg

Extract tips and tricks from the implementation of ffmpeg jn genaiscript: https://github.com/microsoft/genaiscript/blob/main/packages/core/src/ffmpeg.ts

• best code to extract sound, usage of threshold, etc...

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