agent: Support inserting selections as context via @selection (#29045)

WIP

Release Notes:

- N/A

crates/agent/src/assistant_panel.rs

@@ -1951,7 +1951,9 @@ impl AssistantPanelDelegate for ConcreteAssistantPanelDelegate {
                                 .collect::<Vec<_>>();
 
                             for (buffer, range) in selection_ranges {
-                                store.add_excerpt(range, buffer, cx).detach_and_log_err(cx);
+                                store
+                                    .add_selection(buffer, range, cx)
+                                    .detach_and_log_err(cx);
                             }
                         })
                     })