Label chapter chips by kind, not raw number

Beats, tags, locations, open questions, and character/arc-stage
responses that reference a chapter now carry a ChapterLabel/DisplayNumber
alongside the raw Number, computed via the new
ChapterDisplayNumberLookup. Front/back matter chips show their title;
body chapters show "Chapter N: Title".

tests/Novelly.Api.Tests/ListingTests.cs

@@ -85,7 +85,7 @@ public class ListingTests : ServiceTestFixture
         var agent = new NovelAgentService(
             Db.Context,
             new ScriptedModelClient([[new AgentTextBlock("Reply.")]]),
-            new NovelAgentToolset(Novels, Characters, Arcs, Chapters, Beats, Tags, Locations, Questions, NullLogger<NovelAgentToolset>.Instance),
+            new NovelAgentToolset(Novels, Characters, Arcs, Chapters, ChapterLabels, Beats, Tags, Locations, Questions, NullLogger<NovelAgentToolset>.Instance),
             Options.Create(new AgentOptions()),
             NullLogger<NovelAgentService>.Instance,
             new SendAgentMessageRequestValidator());