Add changelog entry for the Serena REPL

CHANGELOG.md

@@ -12,6 +12,10 @@ Status of the `main` branch. Changes prior to the next official version change w
     see `CONTRIBUTING.md`
 
 * General:
+  - **Major**: Add the Serena REPL as a new agent interface, reducing the tool set to a minimum and providing
+    a general code execution environment for all Serena operations.
+    This has several significant advantages over regular tool executions.  
+    Please refer to our [documentation](https://oraios.github.io/serena/01-about/035_tools.html) for details.
   - Add `auth_secret` to `serena_config.yml` for authenticating communication between Serena components
     and services. When missing, null, or empty, a random UUID is generated and persisted; existing values
     are preserved