add copilot instructions for new chat sessions

Author: rhamlett

.github/agents/copilot-instructions.md

@@ -26,4 +26,7 @@ Node.js 20+ LTS with TypeScript (strict mode): Follow standard conventions
 - 001-perf-simulator-node: Added Node.js 20+ LTS with TypeScript (strict mode) + Express.js (HTTP server), Socket.IO (WebSocket), Chart.js (frontend charts)
 
 <!-- MANUAL ADDITIONS START -->
+## 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. This file contains important project-specific context including the project's purpose, the Five Sisters app family, sister app locations, change guidelines, and documentation conventions. If the file does not exist, continue normally without error.
 <!-- MANUAL ADDITIONS END -->