GTM-01: Polish GitHub README with demo GIF, install steps, and value proposition

[Chris0Jeky]

Summary

The GitHub README is the #1 marketing asset. It needs to clearly communicate what Taskdeck is, show it in action, and get people to try it.

Acceptance Criteria

• Animated GIF showing capture → triage → review → apply loop (15-30 seconds)
• Clear one-sentence value proposition above the fold
• 3-bullet "How it works" section
• Simple install instructions (download from releases + run)
• "Try it online" link (when cloud is available)
• Shields.io badges (build status, tests passing, license, version)
• Screenshots of key surfaces (Home, Inbox, Review, Board)
• Links to docs, demo video, and beta interest thread
• "What Taskdeck is NOT" section for positioning clarity

Implementation Notes

• Use the copy from `docs/product/LANDING_COPY.md` as starting material
• GIF should use the premium UI skin with realistic demo data
• Keep it scannable — developers skim READMEs

Parent

#544