docs: record the retention decision and evidence (AUD-75)

docs/retention.md covers the 30-day body window, the actions_log row cap, the
audit-row ordering, configuration, unit + deployed verification, deployment on
.13 (commit 453d8a2), scheduling, and rollback. Phase 7's deferred note now
points at the completed work.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

docs/phase7-summary-hub.md

@@ -290,6 +290,8 @@ The migration-free change started in place on the Phase 1–6 volume
 
 - **1-month body retention** (AGENTS.md §7) is still not implemented. It is a
   data-model rule, not part of the Phase 7 DoD; it remains project backlog.
+  _(Done after this phase in [AUD-75](/AUD/issues/AUD-75); see
+  [`retention.md`](retention.md).)_
 - The Hub's `/notify` receiver does not exist yet in Hub v1, so the outbound
   path is verified against a stub, not the live Hub.
 - `routing.md` still labels `email_agent` a placeholder (see §2.6). The live Hub