add copilot instruction for chat session memory

Author: rhamlett

.github/agents/copilot-instructions.md

@@ -2,6 +2,10 @@
 
 Auto-generated from all feature plans. Last updated: 2026-01-29
 
+## Session Startup
+
+At the start of every new chat session, before answering any questions, read the repository memory file at `/memories/repo/project-context.md` using the memory tool to load project-specific context. If the file does not exist, continue normally without reporting an error.
+
 ## Active Technologies
 
 - C# 12 / .NET 8.0 LTS + ASP.NET Core 8.0, SignalR (real-time dashboard), System.Diagnostics (metrics) (001-perf-problem-simulator)