Diff review comments: store locally and batch submit to agent (#46669)

Doesn't support editing or deleting comments yet, but inputs work:

<img width="797" height="881" alt="Screenshot 2026-01-14 at 11 51 36 AM"
src="https://github.com/user-attachments/assets/746e085b-a919-475a-b804-ee0377e40a0b"
/>


<img width="698" height="178" alt="Screenshot 2026-01-14 at 11 47 51 AM"
src="https://github.com/user-attachments/assets/3edac0e8-62e5-4af5-a324-df2bd293f3ca"
/>

<img width="695" height="695" alt="Screenshot 2026-01-14 at 11 47 58 AM"
src="https://github.com/user-attachments/assets/64795ffd-62b4-48ea-a46c-7724221095d7"
/>


### Features implemented:
- **Local comment storage**: Comments are stored per-hunk in the Editor,
with chronological ordering
- **Expandable comments section**: Shows "N Comments" header that can
expand/collapse
- **Inline editing**: Click edit to transform a comment row into an
editable text field with confirm/cancel buttons
- **Delete functionality**: Remove individual comments
- **Send Review to Agent**: Toolbar button with badge showing comment
count, batch-submits all comments across all files/hunks to the Agent
panel
- **User avatars**: Shows the current user's avatar (from their Zed
account) next to comments, falls back to Person icon when not logged in
- **Visual tests**: Added tests for one comment, multiple comments
expanded, and comments collapsed

Note that this is behind a feature flag, so no release notes yet.

Release Notes:

- N/A

---------

Co-authored-by: Zed Zippy <234243425+zed-zippy[bot]@users.noreply.github.com>
Co-authored-by: David Baldwin <baldwindavid@gmail.com>

crates/git_ui/src/project_diff.rs

@@ -9,7 +9,7 @@ use buffer_diff::{BufferDiff, DiffHunkSecondaryStatus};
 use collections::{HashMap, HashSet};
 use editor::{
     Addon, Editor, EditorEvent, SelectionEffects, SplittableEditor,
-    actions::{GoToHunk, GoToPreviousHunk},
+    actions::{GoToHunk, GoToPreviousHunk, SendReviewToAgent},
     multibuffer_context_lines,
     scroll::Autoscroll,
 };
@@ -70,6 +70,7 @@ pub struct ProjectDiff {
     workspace: WeakEntity<Workspace>,
     focus_handle: FocusHandle,
     pending_scroll: Option<PathKey>,
+    review_comment_count: usize,
     _task: Task<Result<()>>,
     _subscription: Subscription,
 }
@@ -311,8 +312,16 @@ impl ProjectDiff {
                 });
             diff_display_editor
         });
-        cx.subscribe_in(&editor, window, Self::handle_editor_event)
-            .detach();
+        let editor_subscription = cx.subscribe_in(&editor, window, Self::handle_editor_event);
+
+        let primary_editor = editor.read(cx).primary_editor().clone();
+        let review_comment_subscription =
+            cx.subscribe(&primary_editor, |this, _editor, event: &EditorEvent, cx| {
+                if let EditorEvent::ReviewCommentsChanged { total_count } = event {
+                    this.review_comment_count = *total_count;
+                    cx.notify();
+                }
+            });
 
         let branch_diff_subscription = cx.subscribe_in(
             &branch_diff,
@@ -363,8 +372,12 @@ impl ProjectDiff {
             multibuffer,
             buffer_diff_subscriptions: Default::default(),
             pending_scroll: None,
+            review_comment_count: 0,
             _task: task,
-            _subscription: branch_diff_subscription,
+            _subscription: Subscription::join(
+                branch_diff_subscription,
+                Subscription::join(editor_subscription, review_comment_subscription),
+            ),
         }
     }
 
@@ -453,6 +466,16 @@ impl ProjectDiff {
         }
     }
 
+    /// Returns the total count of review comments across all hunks/files.
+    pub fn total_review_comment_count(&self) -> usize {
+        self.review_comment_count
+    }
+
+    /// Returns a reference to the splittable editor.
+    pub fn editor(&self) -> &Entity<SplittableEditor> {
+        &self.editor
+    }
+
     fn button_states(&self, cx: &App) -> ButtonStates {
         let editor = self.editor.read(cx).primary_editor().read(cx);
         let snapshot = self.multibuffer.read(cx).snapshot(cx);
@@ -1278,6 +1301,7 @@ impl Render for ProjectDiffToolbar {
         };
         let focus_handle = project_diff.focus_handle(cx);
         let button_states = project_diff.read(cx).button_states(cx);
+        let review_count = project_diff.read(cx).total_review_comment_count();
 
         h_group_xl()
             .my_neg_1()
@@ -1419,6 +1443,25 @@ impl Render for ProjectDiffToolbar {
                             })),
                     ),
             )
+            // "Send Review to Agent" button (only shown when there are review comments)
+            .when(review_count > 0, |el| {
+                el.child(vertical_divider()).child(
+                    Button::new(
+                        "send-review",
+                        format!("Send Review to Agent ({})", review_count),
+                    )
+                    .icon(IconName::ZedAssistant)
+                    .icon_position(IconPosition::Start)
+                    .tooltip(Tooltip::for_action_title_in(
+                        "Send all review comments to the Agent panel",
+                        &SendReviewToAgent,
+                        &focus_handle,
+                    ))
+                    .on_click(cx.listener(|this, _, window, cx| {
+                        this.dispatch_action(&SendReviewToAgent, window, cx)
+                    })),
+                )
+            })
     }
 }