docs: record check-in goals

The product, storage and design records now say what a goal is, why it
never marks an event done, and why an unreadable goal is kept rather than
stripped. That last rule is the one a future cleanup would most likely
undo by mistake.

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

docs/superpowers/specs/2026-10-03-check-in-goals-design.md

@@ -2,7 +2,7 @@
 
 **Date:** 2026-10-03, revised and approved 2026-10-13
 
-**Status:** approved; spec 1 of 2. Spec 2 is
+**Status:** implemented 2026-10-13; spec 1 of 2. Spec 2 is
 `2026-10-13-periodic-tasks-design.md`, which reuses the period engine defined here.
 
 ## Request and intended outcome