docs(index): track .claude/skills/sister-project-history — the OtOpcUa/HistorianGateway narratives split out in c72271a were gitignored and existed only locally

.gitignore ignored all of .claude/, so the per-project history files the root index points at were never committed; the pushed index had lost that history. Un-ignore .claude/skills/ (settings.local.json and worktrees stay ignored) and add the three files.

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

.claude/skills/sister-project-history/SKILL.md

@@ -0,0 +1,16 @@
+---
+name: sister-project-history
+description: Use when you need the detailed change history, live-gate records, redundancy/auth/ACL behaviour, or ScadaBridge consumer-impact notes for a sister project (OtOpcUa, HistorianGateway, BTDB) — the scadaproj root CLAUDE.md keeps only a short summary row per project; the full narrative lives in this directory.
+---
+
+# Sister project history
+
+The root `CLAUDE.md` index keeps one short row per sister project. The full narrative rows that used to live
+in that table were moved here **verbatim** on 2026-09-03 (they were ~57k characters loading into every session):
+
+- `otopcua.md` — OtOpcUa: v3 dual-namespace address space, every driver, redundancy/mesh program, data-plane auth + ACL enforcement, deferment register, the authoring-vs-deploy-gate parity arc, and the per-issue consumer-impact record for ScadaBridge.
+- `historiangateway.md` — HistorianGateway: ArchReview cycles, pending.md items, session pools, datatype gaps, SQL-path workarounds, PR/commit history.
+- `btdb.md` — BTDB (Batch Tracking Database, added 2026-09-03): schema + sproc write contract, Historian/CLR coupling, Galaxy/MESAPI/ScadaBridge edges, the DARS `BatchTracker_DB` fork, and the gap analysis.
+
+Read the file for the project you are working against. When a sister repo tells you to "update its entry in
+`scadaproj/CLAUDE.md`", append the detail to the file here and keep the index row to a few lines.

.gitignore

@@ -7,8 +7,10 @@ obj/
 [Aa]rtifacts/
 *.user
 
-# Claude Code tooling / agent worktrees (local-only)
-.claude/
+# Claude Code tooling / agent worktrees (local-only) — but .claude/skills/ IS tracked:
+# the sister-project-history skill holds the per-project narratives the root index points at.
+.claude/*
+!.claude/skills/
 
 # Local ops/infra notes — internal access details, never publish
 zimmer-vpn-tunnel.md