Add delete to the selection bar and drop the in-header close button

The selection bar covered copying but not the other action people reach
for with a selection. The header also carried a second close button next
to the window's own traffic light, so it was removed in favor of that.

AGENTS.md

@@ -37,13 +37,14 @@ clicking an item copies it back to the clipboard.
   light mode keeps the system window colour. `Theme.background` is also handed
   to the `NSPanel` so the strip under the transparent titlebar matches, and the
   row colour stays slightly lighter than black so cards remain visible.
-- `Sources/Prompt/PanelView.swift` — the panel UI. Header (section switcher,
-  search, add photos, close), item list, composer. Composer `# Name` creates/
+- `Sources/Prompt/PanelView.swift` — the panel UI. Header (workspace switcher,
+  search, add photos), item list, composer. There is no in-header close button:
+  the window's own close button hides the panel. Composer `# Name` creates/
   switches sections. Click = select (Shift+click for a range); the selection
-  bar offers Copy (single item), Copy as List and Copy as Lines; right-click =
-  context menu acting on the whole selection (Copy, Copy as List, Copy as
-  Lines, Done, Edit, Open, Merge, Move To, Delete). Esc clears the selection
-  before it hides the panel. Drop target for images/files.
+  bar offers Copy (single item), Copy as List, Copy as Lines and Delete;
+  right-click = context menu acting on the whole selection (Copy, Copy as List,
+  Copy as Lines, Done, Edit, Open, Merge, Move To, Delete). Esc clears the
+  selection before it hides the panel. Drop target for images/files.
 - `Sources/Prompt/PanelController.swift` — owns the floating `NSPanel`
   (`.nonactivatingPanel`, `.floating`, right-edge docked, frame autosaved as
   `PromptPanel`), the Cmd+V image-paste monitor, and `EditWindowController`