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/bin/README.md

@@ -0,0 +1,11 @@
+# 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.