agent: add changelog entry for config hash optimization (#3026)

Author: nikw9944

CHANGELOG.md

@@ -8,6 +8,11 @@ All notable changes to this project will be documented in this file.
 
 ### Changes
 
+- Controller
+  - Add `GetConfigHash` gRPC endpoint that returns only the config hash, and include hash in `GetConfig` responses ([#3026](https://github.com/malbeclabs/doublezero/issues/3026))
+- Config Agent
+  - Use hash-based polling to reduce device CPU: check config hash every 5s, only fetch full config when hash changes or every 60s ([#3026](https://github.com/malbeclabs/doublezero/issues/3026))
+
 ## [v0.14.0](https://github.com/malbeclabs/doublezero/compare/client/v0.13.0...client/v0.14.0) - 2026-03-24
 
 ### Breaking