Clarify the opening of the recorded demo

[momonpya]

Summary

Replace the README and demo-guide video with the approved cut whose opening says “Eggshell — AI memory that saves tokens.” Update the video SHA-256 in the measurement record to match that upload.

Why

The opening should identify the product and its purpose immediately. Only the first two seconds of editorial copy changed; the 29-second duration, recorded CLI work, handoff emphasis, token totals, and final comparison are unchanged.

Verification

• Approved MP4: 1920×1080, 24 fps, H.264/AAC, exactly 29 seconds; complete decode reported no errors.
• Reviewed the rendered opening, handoff, and comparison frames.
• Timeline and measurement files match the previous cut byte for byte.
• git diff --check passed; the repository diff is limited to three video URLs and the video hash.
• Documentation and measurement limits remain accurate.
• Performance claims retain inclusive input-plus-output tokens and static answer-review limits.
• All four Linux/macOS Lean and adapter CI checks passed.