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/0059_duplicate_groups.up.sql

@@ -1,9 +1,9 @@
 -- 0059_duplicate_groups.up.sql — proposed duplicates and the sweeps that find
--- them (Scribe milestone #400: #3910).
+-- them.
 --
--- The sweep compares fingerprints (track_fingerprints, 0058) and proposes groups
--- of tracks that hold one recording. Nothing here merges anything: a group is a
--- proposal the operator reviews, and the merge (#3911) is a separate act.
+-- The sweep compares fingerprints (track_fingerprints, 0058) and proposes
+-- groups of tracks that hold one recording. Nothing here merges anything: a
+-- group is a proposal an admin reviews, and the merge is a separate act.
 
 -- One row per sweep. Lets the report tell "the sweep has never run" apart from
 -- "it ran and found nothing", and gives the in-flight guard something to check,
@@ -49,6 +49,6 @@ CREATE TABLE duplicate_group_members (
 CREATE INDEX duplicate_group_members_track_idx ON duplicate_group_members (track_id);
 
 -- The sweep streams candidates in (duration_ms, id) order, keyset-paged, so it
--- only ever holds a few seconds' worth of durations in memory. Without this each
--- page would sort the whole library again.
+-- only ever holds a few seconds' worth of durations in memory. Without this
+-- each page would sort the whole library again.
 CREATE INDEX tracks_duration_id_idx ON tracks (duration_ms, id);