feat(review): group changed files by class with +/- counts and large-group collapse

[JSONbored]

Pure aggregation for the changed-files summary: group the classified files (from the classifier slice) by class, sum additions/deletions per group, and collapse a same-class group above a threshold into one summary row. Data-shape only — the markdown table render is a separate slice.

Deliverables

• Pure summarizeChangedFiles(files: {path, additions, deletions, class}[]): ChangedFileGroup[] returning per-class {class, fileCount, additions, deletions, collapsed}
• Collapse a group into a single row when fileCount exceeds a small constant (documented threshold); otherwise keep per-file rows
• Deterministic ordering (source first, matching diffFilePriority intent)
• Tests: mixed classes, one class over/under the collapse threshold (both branches), zero-add/zero-del files, empty input → []

References

• src/review/review-diff.ts:16
• src/review/review-diff.ts:25
• part of #1957

Part of #1957.

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

[linear-code]

JSO-2246