Support searching across all customization types

When managing chat customizations, it is difficult to find a known item without remembering its type. The Customizations editor currently requires searching each tab separately (for example skills, MCP servers, plugins, and tools).

Add a global search experience that searches across all customization types so users can quickly find and enable or disable an item without first knowing which category it belongs to.

### Expected behavior

- A single search entry point searches every customization type.
- Results identify each item's customization type.
- Users can enable or disable matching customizations directly or navigate to the relevant item.

Start in the Customizations editor and trace the existing per-tab search entry points for skills, MCP servers, plugins, and tools. Implement a single search entry point whose results identify their customization type and support enable/disable or navigation, then verify each expected behavior across all listed types.

Written by the indexing model from the issue text.