Add the ability to follow the agent as it makes edits (#29839)

Nathan here: I also tacked on a bunch of UI refinement.

Release Notes:

- Introduced the ability to follow the agent around as it reads and
edits files.

---------

Co-authored-by: Nathan Sobo <nathan@zed.dev>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>

crates/assistant_tools/src/edit_file_tool.rs

@@ -15,7 +15,7 @@ use language::{
     language_settings::SoftWrap,
 };
 use language_model::{LanguageModelRequestMessage, LanguageModelToolSchemaFormat};
-use project::Project;
+use project::{AgentLocation, Project};
 use schemars::JsonSchema;
 use serde::{Deserialize, Serialize};
 use std::{
@@ -164,6 +164,19 @@ impl Tool for EditFileTool {
                 })?
                 .await?;
 
+            // Set the agent's location to the top of the file
+            project
+                .update(cx, |project, cx| {
+                    project.set_agent_location(
+                        Some(AgentLocation {
+                            buffer: buffer.downgrade(),
+                            position: language::Anchor::MIN,
+                        }),
+                        cx,
+                    );
+                })
+                .ok();
+
             let snapshot = buffer.read_with(cx, |buffer, _cx| buffer.snapshot())?;
 
             if input.old_string.is_empty() {
@@ -226,6 +239,7 @@ impl Tool for EditFileTool {
             let snapshot = cx.update(|cx| {
                 action_log.update(cx, |log, cx| log.track_buffer(buffer.clone(), cx));
 
+                let base_version = diff.base_version.clone();
                 let snapshot = buffer.update(cx, |buffer, cx| {
                     buffer.finalize_last_transaction();
                     buffer.apply_diff(diff, cx);
@@ -233,6 +247,21 @@ impl Tool for EditFileTool {
                     buffer.snapshot()
                 });
                 action_log.update(cx, |log, cx| log.buffer_edited(buffer.clone(), cx));
+
+                // Set the agent's location to the position of the first edit
+                if let Some(first_edit) = snapshot.edits_since::<usize>(&base_version).next() {
+                    let position = snapshot.anchor_before(first_edit.new.start);
+                    project.update(cx, |project, cx| {
+                        project.set_agent_location(
+                            Some(AgentLocation {
+                                buffer: buffer.downgrade(),
+                                position,
+                            }),
+                            cx,
+                        );
+                    })
+                }
+
                 snapshot
             })?;