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

Cargo.lock

@@ -658,6 +658,7 @@ dependencies = [
  "assistant_tool",
  "chrono",
  "gpui",
+ "project",
  "schemars",
  "serde",
  "serde_json",