docs(claude): note the umbrella index's short-row convention for cross-repo propagation

Claude-Session: https://claude.ai/code/session_01Wzg6NUCWHipSexLs2cgRFv

CLAUDE.md

@@ -49,7 +49,11 @@ mxaccessgw changes here — remote/push status, the `.proto` contracts (`mxacces
 `mxaccess_worker.proto`, `galaxy_repository.proto`), the two-process architecture, shared-lib
 consumption, or per-project commands — update the **MxAccessGateway entry in
 [`../scadaproj/CLAUDE.md`](../scadaproj/CLAUDE.md)** in the same change so the umbrella index
-never drifts from this repo. (Mirrors the same rule in the peer repos.)
+never drifts from this repo. (Mirrors the same rule in the peer repos.) **Keep the index row to
+a few lines** — since 2026-09-03 the umbrella keeps only short summary rows and parks long
+per-project narrative in `../scadaproj/.claude/skills/sister-project-history/<project>.md`
+(create `mxaccessgateway.md` there if this gateway ever needs one) and shared-library history
+in `../scadaproj/<lib>/CLAUDE.md`.
 
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