Add demo video + social post links to README (hackathon submission)

Fills the two README placeholders required for the Build Small hackathon submission:
- Demo video: https://www.youtube.com/watch?v=7l51vC20esM
- Social post: https://x.com/ooi_yee_fei/status/2066884944480387503

No other content changed.

README.md

@@ -136,8 +136,8 @@ source-agnostic.
 
 ## Demo video
 
-*(YouTube link to be added)*
+▶️ **[Watch the Kith demo on YouTube](https://www.youtube.com/watch?v=7l51vC20esM)**
 
 ## Social post
 
-*(link to be added)*
+📣 **[Read the launch post on X](https://x.com/ooi_yee_fei/status/2066884944480387503)**