feat: add plugin-create skill and plugin template

Adds the plugin-create skill (.agents/skills/plugin-create/) that scaffolds
a new plugin from templates/plugin/, substitutes PLUGIN_NAME/AUTHOR_* markers,
registers the plugin in marketplace.json, runs claude plugin validate ., and
hands off to /marketplace-architect. Includes evals and the generic template
with all components (agents, skills, hooks, MCP, bin, docs) with per-directory
READMEs explaining what goes where and which tool reads each directory.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

templates/plugin/README.md

@@ -0,0 +1,42 @@
+# PLUGIN_NAME
+
+PLUGIN_DESCRIPTION
+
+## Install
+
+**Claude Code:**
+
+```bash
+claude plugin marketplace add <owner>/<repo>
+claude plugin install PLUGIN_NAME@<marketplace-name>
+```
+
+**GitHub Copilot CLI:**
+
+```bash
+copilot plugin marketplace add <owner>/<repo>
+copilot plugin install PLUGIN_NAME
+```
+
+**Local (development):**
+
+```bash
+# Claude Code
+claude --plugin-dir ./plugins/PLUGIN_NAME
+
+# GitHub Copilot CLI
+copilot plugin install ./plugins/PLUGIN_NAME
+```
+
+## Contents
+
+| Component | Path | Description |
+|---|---|---|
+| Skills | `skills/` | Slash commands available after install |
+| Agents | `agents/` | Role-based agents (`.md` for Claude, `.agent.md` for Copilot) |
+| Hooks | `hooks/hooks.json` (Claude) / `hooks.json` (Copilot) | Event-triggered automation |
+| MCP servers | `.mcp.json` | Model Context Protocol server definitions |
+
+## Author
+
+AUTHOR_NAME