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/0086_library_album_matches.up.sql

@@ -1,10 +1,8 @@
--- Every library album is matched to MusicBrainz before it goes on the list
--- (#5569, operator 2026-10-10: "they should all exist in music brainz we
--- need to use their meta data or a search system to link them up"). The
--- listing in 0085 took only albums whose tags already named their release
--- group and artist; the rest are now matched by the release their tags
--- name, the artist's discography, or a search, and checked against their
--- own tracks.
+-- Every library album is matched to MusicBrainz before it goes on the list, so
+-- its metadata comes from MusicBrainz. The listing in 0085 took only albums
+-- whose tags already named their release group and artist; the rest are now
+-- matched by the release their tags name, the artist's discography, or a
+-- search, and checked against their own tracks.
 --
 --   library_album_matches  one row per album the listing has decided:
 --                          listed (with how it was matched) or unmatched