docs: migration headers keep the before and after, without ids, quotes or one library's numbers (#6277)

Applies rule 32's migration exception: each header still says what the
schema was, what changes and what it prevents, but task, milestone,
decision and rule ids are replaced by the practice they stood for, the
operator quotes by the decision, single-install counts are dropped, and
pointers to a deleted package and an external design doc are gone.
golang-migrate keeps no checksum, so editing applied migrations'
comments is safe.

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

internal/db/migrations/0057_songs_like_tuning.up.sql

@@ -1,13 +1,9 @@
--- 0057_songs_like_tuning.up.sql — a THIRD weight profile, for Songs-like
--- (Scribe #3881, milestone #398).
+-- 0057_songs_like_tuning.up.sql — a THIRD weight profile, for Songs-like.
 --
--- Songs-like shared the `daily_mix` profile with For-You, and that is the bug.
--- The two surfaces want opposite things: For-You is a broad "what will they
--- enjoy today", Songs-like answers "what sounds like THIS", and under one set
--- of weights the broad answer wins. Operator, 2026-09-10: "I'm expecting to
--- get a consistent sound and style from the experience... I was getting a
--- seeming wide variety of music from each one when I was hoping to stay in a
--- certain neighborhood."
+-- Before: Songs-like shared the `daily_mix` profile with For-You. The two
+-- surfaces want opposite things: For-You is a broad "what will they enjoy
+-- today", Songs-like answers "what sounds like THIS" and should stay in one
+-- neighbourhood of sound, and under one set of weights the broad answer wins.
 --
 -- Under the shared daily_mix weights, an UNRELATED track the user had liked
 -- and not played recently scored 1.0 + 2.0 + 1.0 = 4.0 before taste, while a