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/agents/PLUGIN_NAME.agent.md

@@ -0,0 +1,13 @@
+---
+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`.