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/plugin.json

@@ -0,0 +1,11 @@
+{
+  "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"
+}