docs(readme): point existing users to `codegraph upgrade` under the 1.0 banner (#866)

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -4,6 +4,8 @@
 
 ## 🎉 1.0 Released!
 
+Already installed? Run `codegraph upgrade` to update in place.
+
 Follow [@getcodegraph](https://x.com/getcodegraph) on X for updates.
 
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