#1300 land: bounded-mutation-diffs

Accepted agent delivery; branch-keyed search descriptor included.

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Summary by cubic

Bounds optional diff metadata in patch, edit, write, and approval replies with a shared 8 MiB wire budget. Oversized patches are now omitted while file outcomes and line counts stay complete, marked by diff_omitted and a notice; small patches on large files still arrive unchanged.

• Pure additions and deletions are preflighted before line-diff construction, so huge files never render a patch.
• Every reply shape now carries diff_omitted and diff_notice; per-file entries drop patch and set diff_omitted instead.