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/0069_acoustid_lookup.up.sql

@@ -1,11 +1,11 @@
 -- 0069_acoustid_lookup.up.sql — recording MBIDs looked up through AcoustID
--- for tracks whose tags carry none (Scribe milestone #401).
+-- for tracks whose tags carry none.
 --
 -- A track with no recording MBID is invisible to the ListenBrainz similarity
 -- arm in both directions, so this fills the ids the tags leave empty.
 
--- Where tracks.mbid came from. The file's own tag always outranks a lookup
--- (M401 D4): the scan's upsert keeps a looked-up id while the tag carries
+-- Where tracks.mbid came from. The file's own tag always outranks a lookup:
+-- the scan's upsert keeps a looked-up id while the tag carries
 -- none, and replaces it the moment the tag carries one.
 ALTER TABLE tracks ADD COLUMN mbid_source text;
 -- Every id stored so far was read from a tag (the scan or the tag backfill),
@@ -25,7 +25,7 @@ ALTER TABLE tracks
 --                after disambiguating by title and length; recording_mbid is it
 --   ambiguous    several recordings cleared it and nothing told them apart.
 --                Nothing is written: a wrong MBID would feed similarity the
---                wrong neighbours, which is worse than none (M401 D5)
+--                wrong neighbours, which is worse than none
 --   no_match     AcoustID linked nothing at or above the threshold
 --   failed       fpcalc rejected the file, or AcoustID rejected its print
 -- Every state is settled until the file changes. A failure that says nothing
@@ -50,12 +50,12 @@ CREATE TABLE track_acoustid_lookups (
 );
 CREATE INDEX track_acoustid_lookups_state ON track_acoustid_lookups (state);
 
--- AcoustID's knobs, in admin Settings (rule 25). Singleton in the style of
+-- AcoustID's knobs, in admin Settings. Singleton in the style of
 -- loudness_settings (0065).
 CREATE TABLE acoustid_settings (
     id         boolean     PRIMARY KEY DEFAULT true,
-    -- Off by default (rule 26): AcoustID needs a key the operator registers,
-    -- and the lookup sends fingerprints to a third party.
+    -- Off by default: AcoustID needs a key an admin registers, and the lookup
+    -- sends fingerprints to a third party.
     enabled    boolean     NOT NULL DEFAULT false,
     api_key    text,
     -- The score a recording needs before it is considered at all. AcoustID