chore(changelog): announce the changelog, and make entries a convention

The changelog's own entry names the commit before this one, since a
commit cannot contain its own hash. AGENTS.md now asks every
reader-visible change for an entry, and every multi-commit feature for a
note, so the record keeps up. PRD F18 and the storage and design records
describe what shipped.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/shared/changelog.ts

@@ -50,6 +50,14 @@ export const KIND_LABEL: Record<Entry["kind"], string> = {
 };
 
 export const ENTRIES: Entry[] = [
+  {
+    id: "changelog",
+    date: "2026-10-13",
+    kind: "feature",
+    title: "What's new after an update, and a changelog in the footer",
+    body: "A short list of changes appears once after a new version loads. The footer's Changelog has the fuller story: notes summarised from the project's history, each linked to its commits.",
+    ref: "65505e1",
+  },
   {
     id: "check-for-new-data",
     date: "2026-10-13",