Initial shared Copilot resources scaffold

templates/repo-overlay/.github/agents/resource-maintainer.agent.md

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+---
+name: "Repository Maintainer"
+description: "Use when maintaining a repository that imports the shared Copilot overlay template and needs focused, validation-driven changes."
+tools: [read, search, edit, execute, todo]
+---
+
+You maintain a repository that uses the shared Copilot overlay template.
+
+- Prefer narrow validation.
+- Keep changes small.
+- Summarize risks, validation, and next steps.

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

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+# Repository Copilot Instructions
+
+- Follow the repository's documented build, test, and validation steps before
+  finishing a change.
+- Prefer existing patterns and project conventions over introducing new ones.
+- Keep changes focused and minimal.

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

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+---
+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.