docs: OPEN-ITEMS.md — one authoritative register of open work

Open state was spread across ROADMAP, CONTEXT.md, four audits/, three runbooks,
per-session REPORT.md files and a chat log. This is the one page to read first:
every row has a state (BLOCKED/READY/WAITING-ON-OPERATOR/WATCHING) and an owner,
and the READY rows are ranked with reasoning.

R-88 is the recommended next task — quiesce's nil-age fail-open stops every app
stack every 5 minutes with no backoff and bypasses the maintenance window, and
its trigger (a PBS read failure) is live given ep0's demonstrated OOM.

CONTEXT.md now records that OPEN-ITEMS.md is authoritative and that REPORT.md is
overwritten per session.

CONTEXT.md

@@ -3,6 +3,10 @@
 > Created with the REUSE.md rollout (2026-07-03). Authoritative history: `hub/CHANGELOG.md` (hub),
 > `website/CHANGELOG.md`, `scripts/CHANGELOG.md`; end-of-task detail in `REPORT.md`.
 
+> **`documentation/backlog/OPEN-ITEMS.md` is the single source of truth for open work** (2026-07-27).
+> Read it first; `ROADMAP.md` holds the full history and reasoning behind each item. **`REPORT.md` is
+> per-session and overwritten — nothing durable may live only there.**
+
 ## Standing rulings
 
 **S-1 — N.5 gains a third leg: architecture docs are same-session coupled (2026-07-26, R-81).**