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>

docs/superpowers/specs/2026-10-13-changelog-design.md

@@ -2,7 +2,8 @@
 
 **Date:** 2026-10-13
 
-**Status:** approved; spec 3. Independent of specs 1 and 2, and announces them.
+**Status:** implemented 2026-10-13; spec 3. The card and sheet landed together; three same-day
+fix entries for bugs no reader ever saw were left out of the backfill. Independent of specs 1 and 2, and announces them.
 
 ## Request and intended outcome