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

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

CLAUDE.md

@@ -39,7 +39,7 @@ Only these directories may exist at the repository root:
 | `docs/` | Documentation |
 | `hooks/` | Shared hooks (if any) |
 | `mcp-servers/` | Shared MCP servers |
-| `plugins/` | All plugins (projman, projman-pmo, project-hygiene) |
+| `plugins/` | All plugins (projman, projman-pmo, project-hygiene, cmdb-assistant) |
 | `scripts/` | Setup and maintenance scripts |
 
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