A reserved key comes to life — the card window's sidebar grows a Labels section, and labels stops being somebody else's

`labels` has been a reserved tracker key since the rewrite: preserved verbatim, never
interpreted, drawn only as an anonymous row in the Details section beside `assignees` and
`due`. The owner's cards claim it for first-party use, so it joins the schema — read
leniently (a list of names, a bare scalar coercing to one, a mapping malformed and
preserved), written canonically (a quoted flow list in the order the user arranged, no
auto-sort), and removed outright when the last label goes, the way an expanded lane drops
`collapsed`.

Identity is case-insensitive and display is case-preserving, so a card carries `bug` once
however many ways the board spells it, and entries the reading cannot name ride through the
write untouched at the tail.

The section sits second, above Details — which is the point rather than a layout preference:
Details is where keys the app does *not* own are shown, and this key just stopped being one.
Rows rather than chips, because the sidebar is twenty-six characters wide. The add field
autocompletes against the board's own used-labels universe, derived from every live and
trashed card with no store beside the files, and says out loud when Return would mint a word
the board has never used.

Writes ride a `.relabel` operation of their own, because the commit composer has said
"Relabel card 'X'" since long before there was a control to press — and now the undo row says
it too.

Claude-Session: https://claude.ai/code/session_014PtZdPwqZuqEDLc6wZMtEy

Kanban/Storage/BoardWriter.swift

@@ -3025,6 +3025,24 @@ public enum WriteOperation: Sendable, Equatable, CustomStringConvertible {
     case migrateTombstone(title: String?)
     case style(title: String?)      // updateIndex on behalf of styling flows (03-board-ui.md)
 
+    /// **A card's `labels` list being rewritten** — the sidebar section's chips and the context menu's
+    /// submenu, the two surfaces the key's 2026-08-09 activation shipped with
+    /// (`FrontmatterKeys.labels`; `BoardStore.setLabels(_:onCard:on:)`).
+    ///
+    /// **Its own case rather than a fold into `.style`**, on the vocabulary's standing reasoning. A
+    /// restyle picks an appearance; this edits what the card *is about*, and 06-history-undo.md's
+    /// commit-message composer has had a separate word for it since long before there was a control —
+    /// "Relabel card 'X'" (§ Commit messages, named there among the three keys "invisible in the UI
+    /// though they are"). The word already existed; this is the gesture arriving to claim it. A banner
+    /// telling someone the app "couldn't style 'Fix login'" after they clicked a label row would name a
+    /// control they never touched.
+    ///
+    /// **One case for both directions**, unlike `.collapse`/`.expand`: adding and removing a label are
+    /// the same control pressed twice — a chip's ⊗ and a menu row's checkmark both just rewrite the
+    /// list — where collapse and expand are two menu rows with two words. `title` is the card's, filled
+    /// in by `updateIndex` off the document it reads.
+    case relabel(title: String?)
+
     /// **A generated board background landing** — the PNG written into the board folder and the
     /// `background` mapping's two subkeys pointed at it, one bracket
     /// (`BoardStore.applyGeneratedBackground`; `FacetsGenerator`).
@@ -3359,6 +3377,7 @@ public enum WriteOperation: Sendable, Equatable, CustomStringConvertible {
         case .purge: .purge(title: title)
         case .migrateTombstone: .migrateTombstone(title: title)
         case .style: .style(title: title)
+        case .relabel: .relabel(title: title)
         case .resize: .resize(title: title)
         case .collapse: .collapse(title: title)
         case .expand: .expand(title: title)
@@ -3410,7 +3429,7 @@ public enum WriteOperation: Sendable, Equatable, CustomStringConvertible {
         // reading to be about — and `.deleteComment`'s move into `comments/.trash/` stamps for the
         // plain container reason its board-level twin does.
         case .createBoard, .createLane, .createCard, .move, .copy, .delete, .purge, .migrateTombstone,
-             .style, .resize, .collapse, .expand, .rename, .duplicateBoard, .saveAsTemplate, .shareBoard, .paste,
+             .style, .relabel, .resize, .collapse, .expand, .rename, .duplicateBoard, .saveAsTemplate, .shareBoard, .paste,
              .exportBoard, .importBoard,
              .importAttachment,
              .listAttachments, .removeAttachment, .relocateLooseFile, .tidyBackgroundImage, .agentGuide,
@@ -3441,6 +3460,7 @@ public enum WriteOperation: Sendable, Equatable, CustomStringConvertible {
         case let .purge(title): Self.phrase("purge", title)
         case let .migrateTombstone(title): Self.phrase("migrate the legacy 'deleted' key on", title)
         case let .style(title): Self.phrase("style", title)
+        case let .relabel(title): Self.phrase("relabel", title)
         case .setBoardBackground: "set this board's background"
         case let .resize(title): Self.phrase("resize", title)
         case let .collapse(title): Self.phrase("collapse", title)