Initial shared Copilot resources scaffold

templates/repo-overlay/.github/instructions/shared.instructions.md

@@ -0,0 +1,9 @@
+---
+name: "Shared Repository Standards"
+description: "Use when editing files in a repository that imports the shared Copilot overlay template. Covers validation, minimal changes, and reuse of existing patterns."
+applyTo: "**"
+---
+
+- Reuse existing project patterns before introducing new abstractions.
+- Validate changes with the narrowest relevant check available.
+- Keep edits focused on the requested outcome.