feat(review): render changed-files summary as a collapsible table in the unified comment

[JSONbored]

Render the grouped changed-files summary as a collapsible markdown table (class · files · +adds/-dels) inside the unified review comment via the existing extraCollapsibles mechanism. Display gated by a review.changed_files_summary manifest toggle (default off). Deterministic, no AI.

Deliverables

• Add a pure renderer producing the collapsible table markdown, wired as a UnifiedCollapsible (src/review/unified-comment.ts:182) appended via ctx.extraCollapsibles (src/review/unified-comment.ts:505)
• Escape public HTML angles consistent with existing collapsibles (src/review/unified-comment.ts:449)
• Only appended when the toggle is on AND there are changed files; omitted otherwise
• Tests: table markdown for a multi-class PR, collapsed-group row, toggle off → section omitted (both branches), empty → omitted

References

• src/review/unified-comment.ts:182
• src/review/unified-comment.ts:449
• src/review/unified-comment.ts:505
• part of #1957

Part of #1957.

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

[linear-code]

JSO-2247