INST-8 · reviewTaskMetrics collection + Scheduled Trigger + Atlas Charts

> Item derived from the fact-checking cycle analysis (**2026-07-03, v2**). Production numbers (namespace `main`), unless noted.

### Finding (evidence)
There is no dashboard layer; every read today is ad-hoc.

### Action
`reviewTaskMetrics` collection + Scheduled Trigger + Atlas Charts:
- Scheduled aggregation with `$merge` produces one flat doc per task.
- Atlas Charts reads it directly.
- **Must expose the INST-10 (#2505) cross-check `coverageRate` metric.**

### Impact
Live dashboards for cycle, backlog, inter-rater, coverage and volume, filterable by `isCore`.

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- **Priority:** 🟢 Low · **Effort:** M
- **Dependencies:** depends on INST-1..5 being materialized (wave 4). Exposes INST-10 (coverage) and INST-11 (cohort segmentation).
- **Epic:** #2489

Start by reviewing how INST-1..5 are materialized and how the INST-10 (#2505) coverageRate cross-check is exposed. Define the reviewTaskMetrics collection and Scheduled Trigger around the stated $merge output; done means one flat document per task, coverageRate is available, and Atlas Charts can filter live dashboards by isCore.

Written by the indexing model from the issue text.