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/0040_recommendation_tuning.up.sql

@@ -1,11 +1,10 @@
--- Recommendation tuning lab (milestone #127, Scribe #1250).
+-- Recommendation tuning lab.
 --
 -- The scoring weights move out of YAML (radio profile) and out of the
 -- systemMixWeights hard-code (daily_mix profile) into DB-backed
--- settings with live effect — the defaults-discovery lab (decision
--- #1247): the operator turns knobs here to FIND good values, which
--- then get baked into shipped defaults; end users and other operators
--- should never need this card.
+-- settings with live effect — a lab for FINDING good values, which then
+-- get baked into shipped defaults; nobody should need this card to get
+-- good recommendations.
 --
 -- Rows are seeded by the recsettings service's boot reconcile (the
 -- coverart SettingsService pattern), not by this migration, so shipped
@@ -39,7 +38,7 @@ CREATE TABLE taste_tuning (
 );
 
 -- Every knob turn writes one audit row; the metrics trend view
--- (#1251) annotates these on its timeline so cause→effect is visible
+-- annotates these on its timeline so cause→effect is visible
 -- after a change. changes is a jsonb array of {field, old, new}.
 CREATE TABLE recommendation_tuning_audit (
     id         bigserial PRIMARY KEY,