refactor(tui): adopt reactive first index and read-only keyed surface

- quark: Collection.first(name) is now a stable Readable<A | undefined>;
  Keyed.ReadOnly names the read surface; useSlot takes plain constant keys;
  Layout.collectionOf<Target>() replaces the content plan double cast
- BackgroundToolHint reads a backgroundRunning first index instead of
  hand-tracking structure plus tool slots
- toolDisplay moves to util/tool-display (content.ts needs it for the index)

packages/tui/test/cli/tui/inline-tool-wrap-snapshot.test.tsx

@@ -9,8 +9,8 @@ import {
   parseDiagnostics,
   parseQuestionAnswers,
   parseQuestions,
-  toolDisplay,
 } from "../../../src/routes/session"
+import { toolDisplay } from "../../../src/util/tool-display"
 
 let testSetup: Awaited<ReturnType<typeof testRender>> | undefined