Lay the groundwork for a Rust-based eval (#28488)

Also, we moved the logic for driving the agentic loop into `Thread` so
that we don't have to re-implement it.

Release Notes:

- N/A

---------

Co-authored-by: Nathan Sobo <nathan@zed.dev>

crates/eval/README.md

@@ -0,0 +1,7 @@
+# Eval
+
+This eval assumes the working directory is the root of the repository. Run it with:
+
+```sh
+cargo run -p eval
+```