Add reverse-lookup: search which files or examples use a given token or command ID

### Problem
The Tokens and Commands tabs currently support forward discovery, find a token or command, copy its ID, and insert an import or execution snippet. There is no reverse direction: given a token or command ID, find where it is already used across open project files or the bundled extension examples.

When learning the API or debugging a plugin, knowing which existing examples or your own files already use `INotebookTracker` or `notebook:run-cell` is more useful than just knowing the ID exists. This closes the loop between discovery and understanding real usage in context.

### Proposed Solution
A "Find usages" action on each token and command row that searches the current file browser working directory (and optionally the extension-examples folder) for references to that token string or command ID, and surfaces results as a list of file paths with line numbers. or another searchbar, one like the one used in VSCode for "files to include/exclude," but here for reverse lookup

### Additional context
None