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/0007_contextual_likes.up.sql

@@ -1,16 +1,12 @@
--- contextual_likes captures a per-session-context snapshot at the time of
--- each like. The recommendation engine in M3 sub-plan #3 uses these rows
--- to compute contextual_match_score per (current session, candidate track).
+-- contextual_likes captures a per-session-context snapshot at the time of each
+-- like. The recommendation engine uses these rows to compute
+-- contextual_match_score per (current session, candidate track).
 --
 -- Multiple rows per (user, track) are allowed and expected — each row
 -- represents a like in a specific session context. Soft-deleted rows
 -- remain in the table (deleted_at populated) but are filtered out of the
 -- engine's queries via the partial index. Re-liking a previously-unliked
 -- track adds a NEW row (does not undelete the old one).
---
--- Note: the M2 events migration (0005) commented that contextual_likes
--- "ships nullable now" but the actual CREATE TABLE was missed. This slice
--- ships the table for the first time.
 
 CREATE TABLE contextual_likes (
     id              uuid        PRIMARY KEY DEFAULT gen_random_uuid(),
@@ -27,6 +23,6 @@ CREATE INDEX contextual_likes_active_idx
     ON contextual_likes (user_id, track_id)
     WHERE deleted_at IS NULL;
 
--- Vector similarity queries from M3 sub-plan #3 will use this.
+-- For the engine's vector similarity queries.
 CREATE INDEX contextual_likes_vector_idx
     ON contextual_likes USING gin (session_vector);