Create template for custom agent configuration

Added a template for creating a custom agent.

Author: emmanuelknafo

.github/agents/my-agent.agent.md

@@ -0,0 +1,13 @@
+---
+# Fill in the fields below to create a basic custom agent for your repository.
+# The Copilot CLI can be used for local testing: https://gh.io/customagents/cli
+# To make this agent available, merge this file into the default repository branch.
+# For format details, see: https://gh.io/customagents/config
+
+name:
+description:
+---
+
+# My Agent
+
+Describe what your agent does here...