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/0022_user_management.up.sql

@@ -1,14 +1,14 @@
--- User management foundation (U1 of #376):
+-- User management:
 --
 --   - users.display_name: optional human-readable name distinct from username.
 --   - user_invites: admin-generated tokens; user redeems during signup.
 --   - registration_settings (singleton): controls whether registration is
 --     'invite_only' (default after first admin) or 'open' (admin can
---     re-enable). The handler in U1-T2 also has a special-case for the
+--     re-enable). The registration handler also has a special-case for the
 --     "no users yet" state where it ignores invites and lets the first
 --     register as admin.
 --   - audit_log: append-only record of admin-driven user mutations.
---     No retention policy in v1.
+--     No retention policy.
 
 ALTER TABLE users ADD COLUMN IF NOT EXISTS display_name text;