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/0058_track_fingerprints.up.sql

@@ -1,5 +1,4 @@
--- 0058_track_fingerprints.up.sql — an acoustic identity per track (Scribe
--- milestone #400: #3905, #3906).
+-- 0058_track_fingerprints.up.sql — an acoustic identity per track.
 --
 -- A table of its own rather than columns on tracks, for the hot path's sake:
 -- tracks is read with SELECT * by eight queries, among them ListTracksByAlbum,
@@ -16,9 +15,9 @@
 -- A failure that says nothing about the file — a timeout, a cancelled scan, a
 -- missing binary — writes no row at all, so the backfill tries again.
 CREATE TABLE track_fingerprints (
-    -- CASCADE is right here, unlike for the likes and play history M400's
-    -- merge has to carry across: a fingerprint describes one file's bytes and
-    -- means nothing once that file's row is gone.
+    -- CASCADE is right here, unlike for the likes and play history the
+    -- duplicate merge has to carry across: a fingerprint describes one file's
+    -- bytes and means nothing once that file's row is gone.
     track_id            uuid        PRIMARY KEY REFERENCES tracks (id) ON DELETE CASCADE,
     -- SHA-256 of the ENCODED audio packets (ffmpeg -c:a copy -f hash), not of
     -- decoded samples. internal/library/fingerprint.go says why.