docs(readme): use live Discord member-count badge

Replaces the static "Join chat" Discord badge with the live shields.io
discord endpoint that queries the Discord API for the current server
member count. The number is the cortexkit Discord server ID
(1488852091056295957). Style/color/logo kept consistent with the
surrounding badges (flat-square, discord blurple #5865F2).

Author: ualtinok

README.md

@@ -9,7 +9,7 @@
   <a href="https://www.npmjs.com/package/@cortexkit/magic-context"><img src="https://img.shields.io/npm/v/@cortexkit/magic-context?label=cli&color=orange&style=flat-square" alt="npm @cortexkit/magic-context"></a>
   <a href="https://www.npmjs.com/package/@cortexkit/opencode-magic-context"><img src="https://img.shields.io/npm/v/@cortexkit/opencode-magic-context?label=opencode&color=blue&style=flat-square" alt="npm @cortexkit/opencode-magic-context"></a>
   <a href="https://www.npmjs.com/package/@cortexkit/pi-magic-context"><img src="https://img.shields.io/npm/v/@cortexkit/pi-magic-context?label=pi&color=purple&style=flat-square" alt="npm @cortexkit/pi-magic-context"></a>
-  <a href="https://discord.gg/F2uWxjGnU"><img src="https://img.shields.io/badge/Discord-Join%20chat-5865F2?style=flat-square&logo=discord&logoColor=white" alt="Discord"></a>
+  <a href="https://discord.gg/F2uWxjGnU"><img src="https://img.shields.io/discord/1488852091056295957?style=flat-square&logo=discord&logoColor=white&label=Discord&color=5865F2" alt="Discord"></a>
   <a href="https://github.com/cortexkit/magic-context/stargazers"><img src="https://img.shields.io/github/stars/cortexkit/magic-context?style=flat-square&color=yellow" alt="stars"></a>
   <a href="https://github.com/cortexkit/magic-context/commits"><img src="https://img.shields.io/github/last-commit/cortexkit/magic-context?style=flat-square&color=green" alt="last commit"></a>
   <a href="https://github.com/cortexkit/magic-context/blob/master/LICENSE"><img src="https://img.shields.io/badge/license-MIT-green?style=flat-square" alt="MIT License"></a>