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/worktree/src/worktree.rs

@@ -806,6 +806,23 @@ impl Worktree {
         }
     }
 
+    pub fn file_exists(&self, path: &Path, cx: &Context<Worktree>) -> Task<Result<bool>> {
+        match self {
+            Worktree::Local(this) => {
+                let fs = this.fs.clone();
+                let path = this.absolutize(path);
+                cx.background_spawn(async move {
+                    let path = path?;
+                    let metadata = fs.metadata(&path).await?;
+                    Ok(metadata.map_or(false, |metadata| !metadata.is_dir))
+                })
+            }
+            Worktree::Remote(_) => Task::ready(Err(anyhow!(
+                "remote worktrees can't yet check file existence"
+            ))),
+        }
+    }
+
     pub fn load_file(&self, path: &Path, cx: &Context<Worktree>) -> Task<Result<LoadedFile>> {
         match self {
             Worktree::Local(this) => this.load_file(path, cx),