docs: add repo layout structure map

AGENTS.md

@@ -29,6 +29,7 @@ documents:
 - `docs/submodules/08_DECISIONS.md`
 - `docs/submodules/09_ROLLOUT_AND_SSO.md`
 - `docs/submodules/10_ASSET_AND_DOCUMENT_STANDARD.md`
+- `docs/submodules/18_REPO_LAYOUT_STANDARD.md`
 - `docs/submodules/plans/LegacyHUB.md`
 
 If local code, README, or runbook content conflicts with those platform
@@ -51,6 +52,9 @@ Immediate platform implications for this repo:
   API-key checks as service/defense-in-depth auth.
 - OCR/Markdown/Search reuse should converge on a stateless shared core rather
   than duplicating or hard-coding LegacyHUB-specific names into other modules.
+- File-structure work follows `docs/structure-map.md`: current `app/`,
+  `ingestion/`, `indexing` and `storage` layers are mature compatibility shapes
+  and should not be renamed without a scoped migration and focused checks.
 
 ## Stack (canonical)