feat(review): deterministic review-effort/complexity estimator (1-5 band + minutes)

[JSONbored]

Pure estimator turning files-touched, added-line count, file-type mix, and hunk spread into a 1-5 complexity band plus a minutes estimate — the deterministic core of #1955. No AI, no wiring; estimator module + tests only so downstream display/ROI slices stay small.

Deliverables

• New pure module (e.g. src/review/effort-score.ts) estimateReviewEffort(files): {band: 1|2|3|4|5, minutes: number}
• Reuse src/review/review-diff.ts addedLineCount + diffFilePriority and src/signals/path-matchers for the type mix; count hunks via the existing hunk-splitting approach
• Documented, monotonic banding thresholds (bigger/more-scattered diff → higher band)
• Tests: tiny→band1, large multi-file→band5, each threshold boundary (both sides), zero-file guard

References

• src/review/review-diff.ts:16
• src/review/review-diff.ts:25
• src/review/review-diff.ts:33
• part of #1955

Part of #1955.

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

[linear-code]

JSO-2253