PROC-3 · Fact-checker retention/onboarding

> Item derived from the fact-checking cycle analysis (**2026-07-03**). Production numbers (namespace `main`), unless noted.

### Finding (evidence)
39% of fact-checkers do a single activity and never return; the operation depends heavily on a few (1 account concentrates most initial reviews).

### Action
Improve the first-task funnel (mentoring, simpler entry tasks, fast feedback).

### Impact
Broadens the active base and reduces the concentration bottleneck.

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- **Priority:** 🟡 Medium
- **Dependencies:** depends on **INST-7** (onboarding milestones) to measure the effect.
- **Epic:** #2490