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

KanbanTests/BoardWriterTests.swift

@@ -109,7 +109,11 @@ struct BoardWriterPreservationTests {
         #expect(after.contains("title: Renamed\n"))
 
         let document = try FrontmatterDocument.parse(after)
-        #expect(document.unknownFields.map(\.key) == ["project", "sphere", "labels"])
+        // `labels` left this list on 2026-08-09 — it is schema-owned now (`FrontmatterKeys.labels`),
+        // which is exactly why the `keys` and `rawValue` assertions below still name it: the write path
+        // preserves it verbatim like any other key it did not touch, and only its *classification*
+        // moved.
+        #expect(document.unknownFields.map(\.key) == ["project", "sphere"])
         #expect(document.keys == [
             "schema", "title", "order", "project", "sphere", "labels", "created", "modified",
         ])
@@ -1169,7 +1173,8 @@ struct BoardWriterMoveTests {
         #expect(document.created == source.created)
         #expect(document.modifiedBy == .missing)
         #expect(abs(try #require(document.modified.value).timeIntervalSinceNow) < 60)
-        #expect(document.unknownFields.map(\.key) == ["project", "labels"])
+        #expect(document.unknownFields.map(\.key) == ["project"], "`labels` is schema-owned since 2026-08-09")
+        #expect(document.rawValue(for: FrontmatterKeys.labels) == "[a, b, c]", "and still preserved verbatim")
         #expect(after.contains("project: lanework       # agent overlay\n"))
 
         // The move rewrites exactly one file: a destination sibling is not even opened.
@@ -1529,7 +1534,8 @@ struct BoardWriterCopyTests {
         #expect(document.created == sourceDocument.created)
         #expect(document.modifiedBy == .missing)
         #expect(abs(try #require(document.modified.value).timeIntervalSinceNow) < 60)
-        #expect(document.unknownFields.map(\.key) == ["project", "labels"])
+        #expect(document.unknownFields.map(\.key) == ["project"], "`labels` is schema-owned since 2026-08-09")
+        #expect(document.rawValue(for: FrontmatterKeys.labels) == "[a, b, c]", "and still preserved verbatim")
         #expect(document.body == "Card One body — with *markdown*.\n")
 
         // The source is never touched, on any path.