Update changelog for custom agents and /delegate command

Added support for custom agents and a new /delegate command for task delegation.

changelog.md

@@ -1,3 +1,8 @@
+## 0.0.353 - 2025-10-28
+
+- Added support for custom agents. Custom agent definitions are pulled from `~/.copilot/agents`, `.github/agents` in your repository, or your organization's `.github` repository. You can explicitly invoke an agent with the `/agent` slash command interactively or `--agent <agent>` noninteractively. Agents are also provided as tools that the model can call during completion of a task
+- Added a `/delegate` command to delegate a task asynchronously to Copilot coding agent. Any unstaged changes will be committed to a new branch, a PR will be opened in your GitHub repository, and Copilot will complete work in the background.
+
 ## 0.0.352 - 2025-10-27
 
 - Improve handling of MCP tools containing slashes