Description of the 'Open File/URL' Tool

1st chunk of `crates/assistant_tools/src/open_tool/description.md`

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This tool opens a file or URL with the default application associated with it on the user's operating system:
- On macOS, it's equivalent to the `open` command
- On Windows, it's equivalent to `start`
- On Linux, it uses something like `xdg-open`, `gio open`, `gnome-open`, `kde-open`, `wslview` as appropriate

For example, it can open a web browser with a URL, open a PDF file with the default PDF viewer, etc.

You MUST ONLY use this tool when the user has explicitly requested opening something. You MUST NEVER assume that
the user would like for you to use this tool.

Title: Description of the 'Open File/URL' Tool

Summary

This tool allows opening a file or URL using the default application configured in the user's operating system. It functions similarly to the `open` command on macOS, `start` on Windows, and `xdg-open` (or similar) on Linux. It should only be used when the user explicitly requests to open something.