fix: update x handle in readme (#495)

## Description
Updates the Deep Agents README badge to point to the current JavaScript
X handle while preserving the existing visible Follow text casing.

## Test Plan
- [ ] Verify the README X badge links to @langchain_js

_Opened collaboratively by Mason Daugherty and open-swe._

Co-authored-by: open-swe[bot] <open-swe@users.noreply.github.com>
Co-authored-by: Mason Daugherty <61371264+mdrxy@users.noreply.github.com>