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/0038_for_you_pick_kind.up.sql

@@ -1,9 +1,9 @@
--- For You exploration attribution (milestone #127 step 2, Scribe #1249).
+-- For You exploration attribution.
 --
 -- For You is deliberately composed of two populations: a head of
 -- top-scored similarity matches ("taste" picks) and a tail sampled from
 -- deeper in the candidate ranking ("fresh" picks — the freshness
--- injection). Until now the metrics could only judge For You as one
+-- injection). Before this the metrics could only judge For You as one
 -- blob, so a high skip rate couldn't be read as "the taste engine is
 -- missing" vs "the freshness tax is too high".
 --