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/0041_discover_taste_unheard.up.sql

@@ -1,16 +1,13 @@
--- Discover taste-targeted novelty bucket (milestone #127, Scribe #1252/#1488).
+-- Discover taste-targeted novelty bucket.
 --
--- The 2-week metrics review found Discover beating the manual baseline on
--- skip rate — which for a discovery surface means it is playing it safe:
--- on a single-user server it is effectively dormant + crude-random, and
--- the per-user taste profile (n_tags, #796) went unused. This adds a
+-- Without it, Discover on a single-user server is effectively dormant +
+-- crude-random, and the per-user taste profile's tags go unused. This adds a
 -- fourth Discover candidate bucket that ranks unheard tracks by the
 -- user's taste-profile tag weights ("novelty that fits your vibe"), so
 -- its picks need a new pick_kind provenance value.
 --
 -- Postgres CHECK whitelists can't be altered in place: DROP + re-ADD with
--- the expanded list, in the same migration (family rule — new CHECK-enum
--- value needs a same-change migration).
+-- the expanded list, in the same migration.
 ALTER TABLE playlist_tracks
     DROP CONSTRAINT playlist_tracks_pick_kind_check;
 ALTER TABLE playlist_tracks