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/0005_events.up.sql

@@ -1,11 +1,10 @@
--- M2 listening telemetry: play_sessions, play_events, skip_events.
--- Note the table is named play_sessions (not sessions — that table already
+-- Listening telemetry: play_sessions, play_events, skip_events.
+-- The table is named play_sessions (not sessions — that table already
 -- exists from migration 0004 and holds HTTP auth sessions). Listening
--- sessions are an entirely separate concept driven by spec §6.
+-- sessions are an entirely separate concept.
 --
--- session_vector_at_play and contextual_likes ship as nullable now even
--- though M2 writes NULL; M3 will populate them without a follow-up
--- migration.
+-- session_vector_at_play is nullable: plays are recorded before the
+-- recommendation engine fills it.
 
 CREATE TABLE play_sessions (
     id              uuid        PRIMARY KEY DEFAULT gen_random_uuid(),