docs: add animated hero gif to README

[livlign]

Summary

Adds an animated hero GIF at the top of the README to give first-time visitors an at-a-glance sense of what Claude Context does.

The GIF walks through the core pitch in four beats:

1. Millions of lines of code — tiny context window
2. Index once, search semantically
3. One query pulls the right code (shows search_code MCP call with ranked results)
4. Your entire codebase. As Claude's context. (with the real claude mcp add command)

Replaces the existing top-of-README logo (assets/claude-context.png) with assets/hero.gif at the same position. File size: 5.6 MB, 1200×675, 20 s loop.

Test plan

• Open README on github.com/zilliztech/claude-context — hero renders at top and loops cleanly
• README headings, badges, and all following content unchanged
• GIF legible on mobile width