feat: restore cmdb-assistant plugin and fix marketplace paths

- Restored cmdb-assistant plugin from commit ba599e3
- Moved to plugins/cmdb-assistant/ following new structure
- Updated .mcp.json path to ../../mcp-servers/netbox
- Fixed marketplace.json paths for all plugins (now in plugins/)
- Updated README.md with cmdb-assistant link and structure
- Updated CLAUDE.md governance to include cmdb-assistant

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

plugins/cmdb-assistant/.mcp.json

@@ -0,0 +1,9 @@
+{
+  "mcpServers": {
+    "netbox": {
+      "command": "${CLAUDE_PLUGIN_ROOT}/../../mcp-servers/netbox/.venv/bin/python",
+      "args": ["-m", "mcp_server.server"],
+      "cwd": "${CLAUDE_PLUGIN_ROOT}/../../mcp-servers/netbox"
+    }
+  }
+}