INST-4 · Explicit lifecycle state + abandoned/archived

> Item derived from the fact-checking cycle analysis (**2026-07-03, v2**). Refined with backlog decomposition. Absorbs companion **ENG-3 + OPS-2**.

### Finding (refined, verified)
The ~215 open tasks are **not a single block**:
- **~60–75 abandoned** — empty `reviewData`, idle **~590–660 d**
- **~142–155 stuck-active** — have content, idle **~226–235 d**

*Nuance:* in `main` only, the stuck-active drop to **~108 → ~70 d** — the old passive backlog is mostly workshop/test.

### Action
- `lifecycle.status` (draft / in_progress / published / rejected / **abandoned** / **archived**)
- `isOpen`
- `lastActivityAt`
- Daily job that marks `abandoned` by empty `reviewData` **or** N days without activity.

### Impact
The live backlog becomes truly measurable; separates a healthy queue from dead liabilities. Enables the split governance work: PROC-2a (purge/diagnose abandoned) and PROC-2b (triage/SLA stuck-active).

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- **Priority:** 🔴 High · **Effort:** P/M
- **Dependencies:** requires migration/backfill (`needs-migration`). Wave 2, alongside INST-2 (#2492).
- **Epic:** #2489

Start by locating the task model or schema and the existing reviewData handling, then inspect migration/backfill conventions and scheduled-job entry points. Define how lifecycle.status, isOpen, and lastActivityAt interact, and verify that the daily job can distinguish abandoned from archived and active tasks. Done means the migration, state transitions, and measurable backlog behavior are covered.

Rédigé par le modèle d'indexation à partir du texte de l'issue.