Initial shared Copilot resources scaffold

PURPOSE.md

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+# Purpose
+
+This project exists to make reusable Copilot customizations portable,
+maintainable, and easy to evolve.
+
+Without a shared repository, prompts, skills, agents, and instructions tend to
+fragment across user profiles, workspace folders, machines, and shell sessions.
+That makes it difficult to keep behavior consistent or to benefit from what was
+learned in another session.
+
+This repository solves that by keeping shared resources under version control,
+making the repository authoritative after creation, and providing tooling to
+bootstrap, publish, update, and verify the shared environment.