fix: avoid buffering full diff for stats-only git_diff

[Stephenxu000]

Problem

git_diff(include_diff=false) is documented as a stats-only path, but it still materializes the full unified diff before discarding it. Large diffs can therefore exceed the Git child-process output buffer and return spawnSync git ENOBUFS even when raw diff output was explicitly disabled.

Fix

• Use git diff --numstat for the stats-only path instead of generating the unified diff.
• Preserve additions, deletions, changed state, path scoping, staged mode, and Git error diagnostics.
• Keep the existing unified-diff path unchanged when include_diff=true.

Regression coverage

The smoke test creates an isolated Git repository with a diff larger than a 4 KB CODEXPRO_MAX_OUTPUT_BYTES limit. The test fails on the current upstream implementation with spawnSync git ENOBUFS and passes with this change while reporting the expected +600 / -600 stats and an empty raw diff.

Verification

• npm run build
• npm run smoke
• git diff --check

Security impact: none expected. This remains a read-only Git operation and keeps the existing path guard and --no-ext-diff / --no-textconv restrictions.