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/0073_user_notifications.up.sql

@@ -1,11 +1,11 @@
--- M489: a persistent, per-user notifications inbox (#726).
+-- A persistent, per-user notifications inbox.
 --
 -- The event bus is fire-and-forget: a client that is not connected when a
 -- request completes, or when the scanner marks tracks missing, never hears of
 -- it. A row here is the durable record; the bus only nudges open clients to
 -- come and read it.
 --
--- `kind` is CHECK-gated (rule 36). A new kind adds its value here, in the
+-- `kind` is CHECK-gated. A new kind adds its value here, in the
 -- same migration as the code that produces it. The list is mirrored in
 -- internal/notifications/kinds.go, and both CHECKs below must agree with it.