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/.mcp.json

@@ -0,0 +1,3 @@
+{
+  "mcpServers": {}
+}