docs(readme): add demo video preview to project overview

Summary:
- replace the broken inline video embed with a linked README preview image
- add a poster frame asset and update the demo description copy

Rationale:
- keep the demo prominent while using a GitHub rendering path that works
  for this repository
- preserve the checked-in video asset and make the README behavior clear

Tests:
- make -C elixir all

Co-authored-by: Codex <codex@openai.com>

Author: frantic-openai

.github/media/symphony-demo-poster.jpg

@@ -3,6 +3,10 @@
 Symphony turns project work into isolated, autonomous implementation runs, allowing teams to manage
 work instead of supervising coding agents.
 
+[![Symphony demo video preview](.github/media/symphony-demo-poster.jpg)](.github/media/symphony-demo.mp4)
+
+_In this [demo video](.github/media/symphony-demo.mp4), Symphony monitors a Linear board for work and spawns agents to handle the tasks. The agents complete the tasks and provide proof of work: CI status, PR review feedback, complexity analysis, and walkthrough videos. When accepted, the agents land the PR safely. Engineers do not need to supervise Codex; they can manage the work at a higher level._
+
 > [!WARNING]
 > Symphony is a low-key engineering preview for testing in trusted environments.