chore: add changelog entry for MCP get_diff tool

Document the new MCP get_diff capability under Unreleased using the repository changelog format and PR link.

Made-with: Cursor

Author: msukkari

CHANGELOG.md

@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+- Added a `get_diff` MCP tool to return structured diffs between two git refs. [#1142](https://github.com/sourcebot-dev/sourcebot/pull/1142)
+
 ### Changed
 - Synced `vendor/zoekt` with upstream `sourcegraph/zoekt`. [#1140](https://github.com/sourcebot-dev/sourcebot/pull/1140)