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/0045_taste_profile_eras.up.sql

@@ -1,5 +1,5 @@
--- 0045_taste_profile_eras.up.sql — era/decade taste facet (#1530,
--- milestone #160 Opt 2). A third taste facet alongside artists + tags:
+-- 0045_taste_profile_eras.up.sql — era/decade taste facet. A third taste
+-- facet alongside artists + tags:
 -- signed weights over decade buckets ("1990s") derived from
 -- albums.release_date, rebuilt daily by internal/taste next to the
 -- artist/tag facets. Weight is a signed float — positive = drawn to
@@ -18,8 +18,8 @@ CREATE TABLE taste_profile_eras (
 CREATE INDEX taste_profile_eras_user_weight_idx
     ON taste_profile_eras (user_id, weight DESC);
 
--- Era-facet build knob, tunable in the lab (mirrors enriched_tag_scale,
--- #1520): scales how strongly a decade-play imprints on the profile.
+-- Era-facet build knob, tunable in the lab (mirrors enriched_tag_scale):
+-- scales how strongly a decade-play imprints on the profile.
 -- DEFAULT 0.5 backfills the existing singleton and matches
 -- taste.DefaultConfig().EraScale; 0 disables the era facet entirely.
 ALTER TABLE taste_tuning