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/.claude-plugin/plugin.json

@@ -0,0 +1,8 @@
+{
+  "name": "PLUGIN_NAME",
+  "displayName": "PLUGIN_NAME",
+  "description": "PLUGIN_DESCRIPTION",
+  "author": { "name": "AUTHOR_NAME", "url": "AUTHOR_URL" },
+  "license": "MIT",
+  "keywords": []
+}

templates/plugin/.mcp.json

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+{
+  "mcpServers": {}
+}

templates/plugin/README.md

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+# 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

templates/plugin/agents/PLUGIN_NAME.agent.md

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+---
+name: PLUGIN_NAME
+description: PLUGIN_DESCRIPTION
+tools:
+  - read_file
+  - list_directory
+---
+
+Replace this with your agent's system prompt and instructions.
+
+This file is loaded by **GitHub Copilot CLI**. The `tools:` frontmatter field declares
+which tools the agent can use — add or remove tools as needed.
+For Claude Code, see `PLUGIN_NAME.md`.

templates/plugin/agents/PLUGIN_NAME.md

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+---
+name: PLUGIN_NAME
+description: PLUGIN_DESCRIPTION
+---
+
+Replace this with your agent's system prompt and instructions.
+
+This file is loaded by **Claude Code**. For GitHub Copilot CLI, see `PLUGIN_NAME.agent.md`.

templates/plugin/agents/README.md

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+# agents/
+
+Agent definitions for this plugin. Each agent needs two files — one per tool:
+
+| File | Tool | Notes |
+|---|---|---|
+| `<name>.md` | Claude Code | Frontmatter: `name`, `description`. No `tools:` field. |
+| `<name>.agent.md` | GitHub Copilot CLI | Frontmatter: `name`, `description`, `tools:` (array of permitted tool names). |
+
+The two files share the same system prompt body. Keep them in sync.
+
+Rename `PLUGIN_NAME.md` and `PLUGIN_NAME.agent.md` to your agent's name (kebab-case).
+Add additional agent pairs as needed — one pair per agent.

templates/plugin/bin/README.md

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+# bin/
+
+**Claude Code only.** Executables placed here are added to PATH when the plugin is installed.
+
+Use this for CLI tools, helper scripts, or MCP server entry points bundled with the plugin.
+
+Reference files in this directory from `.mcp.json` or hooks using `${CLAUDE_PLUGIN_ROOT}/bin/<file>`.
+The `${CLAUDE_PLUGIN_ROOT}` variable resolves to the plugin's install cache path at runtime —
+do not use relative paths from the repo root, as they will break after install.
+
+GitHub Copilot CLI does not support `bin/`. If you add executables here, they are Claude Code only.

templates/plugin/docs/README.md

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+# docs/
+
+Plugin documentation. Place usage guides, reference material, and examples here.
+
+This directory is not read automatically by either Claude Code or GitHub Copilot CLI.
+Reference specific files from your skill bodies or agent prompts when needed
+(e.g. `See references/usage.md for examples`).

templates/plugin/hooks.json

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+{
+  "hooks": []
+}

templates/plugin/hooks/README.md

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+# hooks/
+
+**Claude Code only.** Hook definitions that run in response to Claude Code events.
+
+The `hooks.json` in this directory is read by Claude Code. Structure:
+
+```json
+{
+  "hooks": {
+    "PostToolUse": [
+      {
+        "matcher": "Bash",
+        "hooks": [
+          { "type": "command", "command": "echo 'tool used'" }
+        ]
+      }
+    ]
+  }
+}
+```
+
+Supported events: `PreToolUse`, `PostToolUse`, `Notification`, `Stop`.
+
+Use `${CLAUDE_PLUGIN_ROOT}` to reference scripts inside this plugin — the plugin runs
+from a cache path after install, not its original repo location.
+
+For GitHub Copilot CLI hooks, see `hooks.json` at the plugin root.

templates/plugin/hooks/hooks.json

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+{
+  "hooks": {}
+}

templates/plugin/plugin.json

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+{
+  "name": "PLUGIN_NAME",
+  "description": "PLUGIN_DESCRIPTION",
+  "author": { "name": "AUTHOR_NAME", "email": "AUTHOR_EMAIL" },
+  "license": "MIT",
+  "keywords": [],
+  "agents": "agents/",
+  "skills": ["skills/"],
+  "hooks": "hooks.json",
+  "mcpServers": ".mcp.json"
+}

templates/plugin/skills/README.md

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+# skills/
+
+Skills for this plugin. Each skill lives in its own subdirectory:
+
+```
+skills/
+  <skill-name>/
+    SKILL.md      # required — frontmatter + skill body
+    META.md       # required — provenance and audit fields
+    references/   # optional — on-demand reference docs
+    scripts/      # optional — executable helper scripts
+    assets/       # optional — templates, data files, lookup tables
+```
+
+**Shared** — both Claude Code and GitHub Copilot CLI read `skills/<name>/SKILL.md`.
+
+To add a skill, run `/write-skill` in a Claude Code session. Do not write SKILL.md by hand
+without following the authoring standard — trigger descriptions and self-checks are required.