feat(memory): backfill retained session transcripts (#115162)

* feat(memory): enumerate retained session transcripts

* feat(memory): add session transcript backfill

* test(memory): cover session backfill workflows

* docs(memory): document session transcript backfill

* fix(memory): preserve oldest session backfill ordering

* fix(memory): harden session archive backfill

* test(memory): prove owner-turn provenance gating

* fix(memory): persist session backfill progress

* fix(memory): preserve ranged backfill progress

* fix(memory): order per-file backfill candidates

* fix(memory): retain paged backfill diary entries

* fix(memory): preserve rem backfill diary history

* fix(memory): adapt backfill to retained session identity

* fix(memory): keep rem backfill preview-only

* fix(memory): satisfy backfill CI guards

docs/docs_map.md

@@ -1757,6 +1757,7 @@ Do not edit it by hand; run `pnpm docs:map:gen`.
   - H2: memory promote-explain
   - H2: memory rem-harness
   - H2: memory rem-backfill
+  - H2: memory session-backfill
   - H2: Dreaming
   - H2: SecretRef gateway dependency
   - H2: Related