docs(core): add plugin README

plugins/core/README.md

@@ -0,0 +1,47 @@
+# core
+
+Cross-cutting utility skills for everyday AI-assisted coding — triage, diagnosis, architecture review, and session navigation.
+
+## Install
+
+**Claude Code:**
+
+```bash
+claude plugin marketplace add <owner>/<repo>
+claude plugin install core@<marketplace-name>
+```
+
+**GitHub Copilot CLI:**
+
+```bash
+copilot plugin marketplace add <owner>/<repo>
+copilot plugin install core
+```
+
+**Local (development):**
+
+```bash
+# Claude Code
+claude --plugin-dir ./plugins/core
+
+# GitHub Copilot CLI
+copilot plugin install ./plugins/core
+```
+
+## Contents
+
+| Component | Path | Description |
+|---|---|---|
+| Skills | `skills/` | Slash commands available after install |
+
+## Skills
+
+| Skill | Description |
+|---|---|
+| `agentsmd-author` | Create or update a repo's AGENTS.md by exploring real build/test/lint conventions; supports nested monorepo placement and hands off to agentsmd-audit and provider-adapter-author |
+| `agentsmd-audit` | Audit a repo's AGENTS.md for embedded secrets, structural completeness, and drift; produces a findings report |
+| `provider-adapter-author` | Convert a provider-specific instruction file (CLAUDE.md, `.cursor/rules/*.mdc`, copilot-instructions.md, etc.) into a thin adapter that defers to AGENTS.md |
+
+## Author
+
+Defame1297