Import ffmpeg shared workflow from github/gh-aw

[Copilot]

Adds the ffmpeg shared workflow component from github/gh-aw into the workflows/shared/ folder, making it available for other workflows in this repo to import.

Changes

• workflows/shared/ffmpeg.md — Copied from github/gh-aw:.github/workflows/shared/ffmpeg.md
  • Installs ffmpeg and ffprobe via setup step
  • Provides bash tool allowlist for ffmpeg * and ffprobe *
  • Documents common operations: audio extraction (MP3/Opus), frame extraction, scene detection, resize/convert, video info via ffprobe
  • Includes stable hash pattern for cache-keying ffmpeg operations

Usage

---
on: workflow_dispatch
imports:
  - shared/ffmpeg.md
---