Build the attachments sidebar section

The card's complete file inventory: compact QuickLook-thumbnail rows
over Card.attachments — no reference tracking, subfolders tolerated
and unsurfaced — with a quiet header add affordance and the drop hint
empty state. The whole window is the file-drop surface, Edit mode
included (the editor's drag types were already filtered; now tested),
sharing the board's folder-refusal semantics literally: FinderDrop
moved verbatim into its own file so both windows run the same
partition and loss row. Dragged text still lands at the caret and is
inert elsewhere — the window delegate accepts file payloads only.
Rows open on double-click or Return, drag out their file URL, and
Remove is a bracketed write through FileManager.trashItem — the system
Trash, never a hard delete, returning the in-Trash URL so the promise
is testable; the attachment listing is the guard, so traversal and
subfolder names refuse in one line. Keyboard-native per 05: the
section is one Tab stop, arrows walk rows by name, Space toggles the
shared QuickLook panel, Backspace removes. File > Add Attachment
(shift-cmd-A) comes alive through the same import path.

Claude-Session: https://claude.ai/code/session_01SR4XGjmBE16ZUYWpfFHXwY

Kanban/App/AppCommands.swift

@@ -322,12 +322,15 @@ private final class DuplicateCancellation {
 /// will drop) disables rather than falling back to the root: revealing the wrong thing is worse
 /// than nothing, and only a genuinely empty selection means "the board".
 ///
-// m6-card-window: the item's third scope — the card's folder, or the selected attachment's file
-// when the attachments section is focused — adds a focused value and a branch here; the two below
-// do not move.
+/// **The card-window scope is the third branch**, and it is the one the attachment row's context
+/// menu twins (11-command-nexus.md ▸ Context menus): "card window: the card's folder — the selected
+/// attachment's file instead when the attachments section is focused". The rule itself is
+/// `CardAttachments.revealURLs`, so the menu row and the row's own Reveal cannot disagree about what
+/// "the selected attachment" means.
 struct RevealInFinderCommand: View {
 
     @FocusedValue(\.boardStore) private var store
+    @FocusedValue(\.cardAttachments) private var attachments
     @FocusedValue(\.welcomeSelection) private var selection
 
     var body: some View {
@@ -338,7 +341,8 @@ struct RevealInFinderCommand: View {
     }
 
     /// What the item would reveal, and therefore whether it is enabled — one answer for both, the
-    /// codebase's usual shape. The board in front wins; the welcome branch stands when no board is.
+    /// codebase's usual shape. The board in front wins; the card-window branch stands when a card
+    /// window is; the welcome branch stands when neither is.
     private var urls: [URL] {
         if let store {
             let ids = store.selection.ids
@@ -346,6 +350,13 @@ struct RevealInFinderCommand: View {
             return TrashModel.paths(of: ids, on: store.selection.liveness, in: store.snapshot)
                 .map { $0.folder(under: store.rootURL) }
         }
+        if let attachments {
+            return CardAttachments.revealURLs(
+                cardFolder: attachments.cardFolder,
+                selectedURL: attachments.selectedURL,
+                isSectionFocused: attachments.isFocused
+            )
+        }
         guard let selection, selection.canReveal, let url = selection.url else { return [] }
         return [url]
     }