chore(r2-12): untrack/archive root planning files; honor pending.md's never-stage rule (07/U-4)

- pending.md, current.md: git rm --cached (untrack, keep on disk) + root-anchored
  .gitignore entries — making the files' own declared 'never stage' rule true.
- looseends.md: deleted (2026-05-18 snapshot; sole open item + all follow-ups
  target the retired Wonderware historian sidecar rig, moot since the gateway
  cutover; recoverable from history).
- stillpending.md -> docs/plans/2026-06-18-stillpending-backlog-audit.md (git mv;
  referenced by docs/Historian.md + memory index, preserved under dated-plan convention).
- HISTORIAN-GATEWAY-INTEGRATION-ISSUES.md -> docs/plans/2026-06-27-historian-gateway-integration-issues.md (git mv).
- docs/Historian.md §'Closed backlog §2 items' repointed at the archived path.
Historical docs/plans/* references to the bare stillpending.md filename are frozen
point-in-time records, left untouched per the documentation-audit convention.

docs/Historian.md

@@ -394,9 +394,10 @@ Array writes, multi-dimensional arrays (ValueRank>1), and per-element historizat
 
 ---
 
-## Closed stillpending.md §2 items (Phase 4 / Phase 4b)
+## Closed backlog §2 items (Phase 4 / Phase 4b)
 
-The following items from the `stillpending.md` backlog §2 were closed by earlier
+The following items from the backlog §2 (archived at
+`docs/plans/2026-06-18-stillpending-backlog-audit.md`) were closed by earlier
 phases and are recorded here so future audits don't re-flag them.
 
 | Item | Closed by | Commit |