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/0025_library_changes.up.sql

@@ -1,13 +1,10 @@
 -- Append-only change log for library entities. Every mutation on
 -- artists/albums/tracks/likes/playlists/playlist_tracks writes a row
--- in the same transaction as the mutation itself. Powers the Flutter
--- delta-sync endpoint (#357) — clients pass back the last-seen `id`
--- as a cursor.
+-- in the same transaction as the mutation itself. Powers the delta-sync
+-- endpoint — clients pass back the last-seen `id` as a cursor.
 --
--- Retention: not capped here. If the table grows unbounded a follow-up
--- adds a periodic compactor (collapse contiguous upserts on the same
--- entity, drop pre-delete history). Cursor reset path (410) covers the
--- compactor's edge.
+-- Retention: not capped here. A cursor older than what the table keeps gets
+-- the reset path (410).
 
 CREATE TABLE library_changes (
   id BIGSERIAL PRIMARY KEY,