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/FrontmatterDocument.swift

@@ -593,6 +593,42 @@ public enum FrontmatterKeys {
     /// the reason it always did — it is Lanework's to interpret, not an unknown key riding along.
     public static let hero = "hero"
 
+    /// **A card's labels** (01-storage-format.md § Frontmatter's card table — the amendment this key's
+    /// activation owes; `CardLabels`, `FrontmatterDocument.labels`) — a YAML list of names:
+    /// `labels: ["bug", "ui"]`.
+    ///
+    /// ### It used to be reserved, and that reversal is the design event
+    ///
+    /// Through 2026-08-09 this was one of the **reserved tracker keys** beside `assignees`, `due`,
+    /// `remote` and `remote-state` (01 § Enhanced schema): "the pathfinder shipped them natively; the
+    /// rewrite reserves them instead", preserved verbatim, never interpreted, and shown only as an
+    /// anonymous row in the card window's Details section like any other unknown key. The owner's cards
+    /// ("in card window sidebar add ability to manage card's labels", "add labels submenu to card
+    /// context menu") claim it for **first-party use**, which retires the reservation for this one name
+    /// — `assignees`, `due` and the two `remote` keys are untouched and stay exactly as reserved as they
+    /// were.
+    ///
+    /// **It therefore joins `schemaOwned`**, which is the whole of what stops the Details section
+    /// drawing a second, raw copy of a field the sidebar now renders properly (`CardDetails.rows`
+    /// subtracts exactly that set). `hero`'s precedent, and for `hero`'s reason: it is Lanework's to
+    /// interpret, not an unknown key riding along.
+    ///
+    /// ### The grammar
+    ///
+    /// **Read leniently, written canonically** — the shape of every lenient field here, one collection
+    /// up. A sequence reads as its string entries in file order; a bare scalar coerces to a
+    /// one-element list (`labels: bug`); a mapping has no list reading at all and is `.malformed`.
+    /// Names are **case-preserving for display and case-insensitively unique within a card**, and
+    /// entries the reading cannot make a name of are preserved on the way out but never rendered
+    /// (`CardLabels`).
+    ///
+    /// **Written as a flow list, in the user's own order** — no auto-sort: the order is the order of
+    /// addition, which is a thing the user arranged and the app has no business restating
+    /// alphabetically. **Removing the last label removes the key**, the remove-at-default family's rule
+    /// beside `collapsed`'s expand and the None well's `background`: an absent key is the no-labels
+    /// reading, so `labels: []` written by the app would be noise every unlabelled card had to carry.
+    public static let labels = "labels"
+
     public static let created = "created"
     public static let modified = "modified"
     public static let modifiedBy = "modified-by"
@@ -620,7 +656,8 @@ public enum FrontmatterKeys {
     public static let kind = "kind"
 
     /// The reserved tracker keys — `remote` (board, card) and `remote-state` (lane) — of the enhanced
-    /// schema's future connectors (01-storage-format.md § Enhanced schema).
+    /// schema's future connectors (01-storage-format.md § Enhanced schema). **`labels` left this
+    /// family on 2026-08-09** when the owner claimed it for first-party use; see that key above.
     ///
     /// **Named here without joining `schemaOwned`**, which is the ruling rather than an oversight: 01
     /// says the app "treats reserved keys as ordinary unknown keys (preserved verbatim, invisible in
@@ -652,7 +689,7 @@ public enum FrontmatterKeys {
     public static let author = "author"
 
     public static let schemaOwned: Set<String> = [
-        schema, title, order, width, collapsed, hero, created, modified, modifiedBy, deleted,
+        schema, title, order, width, collapsed, hero, labels, created, modified, modifiedBy, deleted,
         background, icon, iconColor, kind,
     ]
 }