add CHANGELOG entry for #994

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: msukkari

CHANGELOG.md

@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+- Gate MCP API behind authentication when Ask GitHub is enabled. [#994](https://github.com/sourcebot-dev/sourcebot/pull/994)
+
 ## [4.15.4] - 2026-03-11
 
 ### Added