assistant_tools: Add `list-worktrees` and `read-file` tools (#26147)

This PR adds two new tools to Assistant 2:

- `list-worktrees` - Lists the worktrees in a project
- `read-file` - Reads a file at the given path in the project

I don't see `list-worktrees` sticking around long-term, as when we have
tools for listing files those will include the worktree IDs along with
the path, but making this tool available allows the model to utilize
`read-file` when it otherwise wouldn't be able to.

Release Notes:

- N/A