[demo] Screen recording + English voiceover + edit the 3-min video

[sergi-torres]

Record demo in 1080p (OBS or Loom). Record English voiceover following docs/demo_script.md. Edit to exactly 3 minutes per MVP §3 structure. Upload to YouTube (unlisted). Add link to README.

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Module: demo  ·  Size: L  ·  Sprint: Sprint 3 — Polish + Demo  ·  Priority: P2