docs: update changelog to reflect cross-platform CI and split runner roles

Signed-off-by: Sebastien Tardif <sebtardif@ncf.ca>

Author: SebTardif

CHANGELOG.md

@@ -31,5 +31,6 @@
 ### Infrastructure
 
 - Automated test harness with unit tests and VS Code extension integration tests
-- CI on self-hosted runner
+- Cross-platform CI matrix (Ubuntu, macOS, Windows) for unit tests
+- Self-hosted runner for build, packaging, and integration tests
 - Extension packaging with `@vscode/vsce`