Update project page with hand-control demo

[ryanontheinside]

Summary

• Replace the prompt-blend sample card with the new bird hand-control demo video from R2
• Reframe the card copy around DEMON supporting custom MediaPipe/performance interfaces
• Add a poster still generated from the new video

Checks

• Served docs locally at http://127.0.0.1:8080/ and confirmed the new video URL/copy are present and the old prompt-blend sample URL/title are absent
• Verified the generated poster image visually

[viborc]

LGTM ✅

Reviewed locally (served docs/ and compared against the live page). Checked each claim:

• New video is live. bird_demo_hands.mp4 on the R2 bucket returns 200 (23.8 MB, uploaded same day as the commit).
• The poster is valid. poster-bird-demo-hands.jpg, 1920×1080, matches the video.
• Old card fully removed. No leftover prompt-blend references except the unrelated LLM-agent caption that mentions "prompt-blend updates" as a control parameter, which is correct and not a stray.
• Copy is accurate. The "separate MediaPipe hand-tracking frontend maps gestures onto DEMON's live controls" line checks out. The apps/summon demo in demon-example-apps mounts as an external static demo via the shared /sdk/ bundle and uses MediaPipe hand tracking as a control surface (see demos/realtime_motion_graph_web/README.md).
• Base branch is correct. Targets docs/project-page, which is the GitHub Pages publishing branch (/docs path), so merging publishes straight to the live page.

There's one non-blocking nit: docs/assets/img/poster-prompt-blend.jpg is now orphaned. This PR removes its only reference but leaves the file. Fine to drop here or clean up later.

Your choice.