feat(demo): add co-dialectic v4.6.1 product demo video

42s marketing video: 3-slide hook (stop search engine → stop
assistant/boss → co-thinking partner) + 8 product demo frames.

Includes source HTML, stitch.py pipeline, frame PNGs, and output MP4.
940x680, 24fps, 679KB. Ready for Gemini enhancement pass.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: thewhyman