Add Changelog entry

Author: Gavin Williams

CHANGELOG.md

@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+- [Experimental] Added customisable agent configs for the AI code review agent, supporting per-repo, per-connection, and org-wide configuration of prompt, model, review command, auto-review behaviour, and context files. [#1149](https://github.com/sourcebot-dev/sourcebot/pull/1149)
+
 ## [4.16.15] - 2026-04-23
 
 ### Changed