chore: update CHANGELOG for AI SDK v6 upgrade

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

Author: brendan-kellam

CHANGELOG.md

@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+- Upgraded Vercel AI SDK from v5 to v6 (`ai`, all `@ai-sdk/*` packages, `@openrouter/ai-sdk-provider`). [#969](https://github.com/sourcebot-dev/sourcebot/pull/969)
+
 ## [4.13.1] - 2026-02-28
 
 ### Changed