docs: add sanitized Huddle demo recording

[alexdancer]

Demo preview

• GIF on this branch
• Direct raw GIF
• Rendered README on this branch

What changed

• Adds a primary 20-second silent GIF walkthrough and a lightweight MP4 fallback.
• Starts the public recording at the Evidence Desk and excludes local file-picker/notification frames.
• Uses company-agnostic education, teacher, and guide framing while retaining the full-stack delivery, adaptive-academic workflow, synthetic-data safeguards, evidence-backed ranking, bounded AI guidance, and deployment documentation.
• Adds a synthetic Evidence Desk screenshot that shows the ranked attention queue and teacher action guidance.

Validation

• GIF (2.4 MB) and MP4 fallback (1.0 MB) decode successfully; each is under 10 MB.
• Sampled frames and OCR scans contain no company-specific, credential, or private-notification text.
• README local paths resolve; npx prettier --check README.md and git diff --check pass.