feat: add kyberforge plugin and rename plugin-create skill

Scaffolds the kyberforge marketplace management plugin (create-plugin and
marketplace-architect skills will live here). Renames the plugin-create
skill directory to create-plugin to match the canonical kebab-order convention.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.agents/skills/create-plugin/SKILL.md

@@ -1,5 +1,5 @@
 ---
-name: plugin-create
+name: create-plugin
 description: >
   Use when the user wants to create a new plugin in the marketplace — scaffold the directory
   structure, generate plugin.json manifests for Claude Code and Copilot CLI, and register