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

crates/assistant_tools/src/assistant_tools.rs

@@ -1,13 +1,19 @@
+mod list_worktrees_tool;
 mod now_tool;
+mod read_file_tool;
 
 use assistant_tool::ToolRegistry;
 use gpui::App;
 
+use crate::list_worktrees_tool::ListWorktreesTool;
 use crate::now_tool::NowTool;
+use crate::read_file_tool::ReadFileTool;
 
 pub fn init(cx: &mut App) {
     assistant_tool::init(cx);
 
     let registry = ToolRegistry::global(cx);
     registry.register_tool(NowTool);
+    registry.register_tool(ListWorktreesTool);
+    registry.register_tool(ReadFileTool);
 }