Initial shared Copilot resources scaffold

docs/authoring.md

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+# Authoring Guide
+
+## Choose The Right Primitive
+
+- Use a skill for reusable multi-step workflows, especially if the workflow
+  should work in Copilot CLI as well as VS Code.
+- Use a prompt for a lightweight VS Code slash command.
+- Use an instruction file for conventions or rules that should influence model
+  behavior.
+- Use an agent for a reusable persona with tool restrictions or handoffs.
+- Use a hook only when the behavior must be deterministic and enforced in code.
+
+## Supported Creation Paths
+
+### Repo-first
+
+Open this repository in VS Code and create the resource directly in the matching
+ folder.
+
+### Local-first
+
+Create the resource from a normal VS Code workspace or user-profile flow, then
+publish it into this repository with `install/publish.sh` or
+`install/publish.ps1`.
+
+## Rule Of Record
+
+If a resource is meant to be shared, the version in this repository is the only
+version that counts.