feat(review): review.changed_files_summary manifest toggle + docs

[JSONbored]

Add the per-repo review.changed_files_summary config-as-code toggle (default off) that gates the changed-files summary table, plus the .gittensory.yml docs entry. Small config-plumbing slice so the render slice can stay focused on rendering.

Deliverables

• Resolve review.changed_files_summary from the per-repo manifest layer (mirror the review.inline_comments resolution used by shouldRequestInlineFindings)
• Pure boolean helper defaulting to false when unset
• Doc the toggle in the config-as-code reference alongside the other review.* toggles
• Tests: unset → false, on/off values via the truthy-string regex (all branches)

References

• src/review/inline-comments.ts:21
• src/config/gittensory-repo-focus-manifest.ts
• part of #1957

Part of #1957.

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size: XS · held for later release (gittensor:bug once activated)

[linear-code]

JSO-2248