Show progress indicator for /diff command (#2245)

## Summary
- Show a temporary Working on diff state in the bottom pan 
- Add `DiffResult` app event and dispatch git diff asynchronously

## Testing
- `just fmt`
- `just fix` *(fails: `let` expressions in this position are unstable)*
- `cargo test --all-features` *(fails: `let` expressions in this
position are unstable)*

------
https://chatgpt.com/codex/tasks/task_i_689a839f32b88321840a893551d5fbef

codex-rs/tui/src/bottom_pane/bottom_pane_view.rs

@@ -41,4 +41,8 @@ pub(crate) trait BottomPaneView<'a> {
     ) -> Option<ApprovalRequest> {
         Some(request)
     }
+
+    /// Optional hook for views that expose a live status line. Views that do not
+    /// support this can ignore the call.
+    fn update_status_text(&mut self, _text: String) {}
 }

codex-rs/tui/src/bottom_pane/mod.rs

@@ -210,6 +210,19 @@ impl BottomPane<'_> {
         }
     }
 
+    /// Update the live status text shown while a task is running.
+    /// If a modal view is active (i.e., not the status indicator), this is a no‑op.
+    pub(crate) fn update_status_text(&mut self, text: String) {
+        if !self.is_task_running || !self.status_view_active {
+            return;
+        }
+        if let Some(mut view) = self.active_view.take() {
+            view.update_status_text(text);
+            self.active_view = Some(view);
+            self.request_redraw();
+        }
+    }
+
     pub(crate) fn composer_is_empty(&self) -> bool {
         self.composer.is_empty()
     }

codex-rs/tui/src/bottom_pane/status_indicator_view.rs

@@ -43,4 +43,8 @@ impl BottomPaneView<'_> for StatusIndicatorView {
             self.view.interrupt();
         }
     }
+
+    fn update_status_text(&mut self, text: String) {
+        self.update_text(text);
+    }
 }