docs: add CHANGELOG entry for chat preferences

Adds a new `### Added` heading under `[Unreleased]` (the section
previously only had `### Changed`) for the per-user Chat Preferences
feature.

Refs #1242, #1243

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: adam.wilson

CHANGELOG.md

@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+- Added a per-user Chat Preferences settings page where users can tune the chat agent's response style across six dimensions (depth, code visibility, vocabulary, citation density, output structure, diagrams) plus a free-text custom-instructions field, applied as soft biases to the agent's system prompt. [#1243](https://github.com/sourcebot-dev/sourcebot/pull/1243)
+
 ### Changed
 - Documented session lifetime, repository visibility refresh behavior, and how permission sync handles transient code-host errors. [#1218](https://github.com/sourcebot-dev/sourcebot/pull/1218)