INST-5a · Persist the reviewer's initial classification as a durable field

> Item derived from the fact-checking cycle analysis (**2026-07-03, v2**). Split from INST-5 (#2495). Companion proposal **ENG-2** — its strongest point.

### Finding (evidence)
Cross → final adoption (old 98.7%) is **unauditable** — `report.classification` equals the initial value. The reviewer's initial classification already exists **transiently** in ReviewTask history.

### Action
Persist the reviewer's **initial classification** as a durable field. This is the smallest change with the largest audit payoff, and guarantees the 98.7% storage artifact can never recur.

### Impact
The three-way initial→cross→final reconstruction becomes a first-class query.

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- **Priority:** 🔴 High · **Effort:** P (quick win)
- **Parent:** #2495 · **Epic:** #2489

Start by tracing where ReviewTask history stores the reviewer's initial classification and where report.classification is persisted. Check the parent issue and companion proposal for the intended field and data flow. Done means the initial classification survives beyond task history and supports the initial→cross→final reconstruction query.

索引モデルが issue の本文から書いたものです。