feat(review): surface aggregate review-effort in the self-host maintainer stats feed

[JSONbored]

Expose the aggregate/average review-effort band and estimated minutes in the self-host maintainer stats feed (src/review/stats.ts), so a maintainer triaging the queue gets a complexity read. Maintainer-facing stats only — distinct from the public counter; deterministic.

Deliverables

• Add an aggregate effort field (avg band + total estimated minutes) to the stats payload in src/review/stats.ts
• Compute from the same estimator; degrade to null/0 when there's no data (fail-safe read pattern)
• Keep it inside the maintainer/self-host feed, not the public-stats payload
• Tests: populated aggregate, empty → null/0 (both branches)

References

• src/review/stats.ts
• src/review/public-stats.ts
• part of #1955

Part of #1955.

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size: S · held for later release (gittensor:feature once activated)

[linear-code]

JSO-2256