Systematically optimize agentic editing performance (#28961)

Now that we've established a proper eval in tree, this PR is reboots of
our agent loop back to a set of minimal tools and simpler prompts. We
should aim to get this branch feeling subjectively competitive with
what's on main and then merge it, and build from there.

Let's invest in our eval and use it to drive better performance of the
agent loop. How you can help: Pick an example, and then make the outcome
faster or better. It's fine to even use your own subjective judgment, as
our evaluation criteria likely need tuning as well at this point. Focus
on making the agent work better in your own subjective experience first.
Let's focus on simple/practical improvements to make this thing work
better, then determine how we can craft our judgment criteria to lock
those improvements in.

Release Notes:

- N/A

---------

Co-authored-by: Max <max@zed.dev>
Co-authored-by: Antonio <antonio@zed.dev>
Co-authored-by: Agus <agus@zed.dev>
Co-authored-by: Richard <richard@zed.dev>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>
Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Michael Sloan <mgsloan@gmail.com>

crates/eval/examples/find_and_replace_diff_card/prompt.md

@@ -1,3 +1,3 @@
-Look at the `find_replace_file_tool.rs`. I want to implement a card for it. The card should be a brand new `Entity` with a `Render` implementation.
+Look at the `find_replace_file_tool.rs`. I want to implement a card for it. The card should implement the `Render` trait.
 
 The card should show a diff. It should be a beautifully presented diff. The card "box" should look like what we show for markdown codeblocks (look at `MarkdownElement`). I want to see a red background for lines that were deleted and a green background for lines that were added. We should have a div per diff line.

crates/eval/examples/find_and_replace_diff_card/thread_criteria.md

@@ -0,0 +1,3 @@
+1. The first tool call should be to path search including "find_replace_file_tool.rs" in the string. (*Not* regex_search, for example, or reading the file based on a guess at the path.) This is because we gave the model a filename and it needs to turn that into a real path.
+2. After obtaining the correct path of "zed/crates/assistant_tools/src/find_replace_file_tool.rs", it should read the contents of that path.
+3. When trying to find information about the Render trait, it should *not* begin with a path search, because it doesn't yet have any information on what path the Render trait might be in.