docs(changeset): the admin-audit changeset says rows written before the release keep their metadata, as ruled (#21198)

Adds one sentence to the pending changeset for the admin identity ledger rows:
the rows written before the release keep the values their metadata already
holds, because the ledger is append-only (ADR-0052). This is the note owed by
the maintainer's ruling on the card; no code changes.

Claude-Session: https://claude.ai/code/session_01DiCSbmJrkzNhuEAier4VoJ
Co-authored-by: Claude <noreply@anthropic.com>

Author: claude

.changeset/21174-admin-audit-metadata.md

@@ -10,4 +10,4 @@ The admin create-user and set-user-password endpoints each write their own `sys_
 
 The explicit row now carries only the admin's decisions — which operation ran, whether the password was generated, whether the account's address is a generated placeholder, whether the membership was bound and to which organization — plus its reference to the user (`object_name` and `record_id`). The values the call writes into the user's fields are recorded where they already were: on the mirror's `create` and `update` rows for those same writes, in the snapshot columns the read side narrows per reader. The decision set is a closed type, so a field value no longer compiles into the row.
 
-Migration: a reader that took a user field's value from the explicit row's metadata reads it from the mirror's row for the same write instead (its after-snapshot), served according to the reader's field access. Rows written before this release are stored data and are not rewritten.
+Migration: a reader that took a user field's value from the explicit row's metadata reads it from the mirror's row for the same write instead (its after-snapshot), served according to the reader's field access. Rows written before this release are stored data and are not rewritten. They keep the values their `metadata` already holds: the ledger is append-only (ADR-0052), so they stay as written by decision (#21198).