fix: summarize oversized git diffs

[RealBhupesh]

Closes #770
Part of #772

Summary

• Route oversized non-stat git_diff results through git diff --stat instead of returning the full patch.
• Keep the existing 500-line safety threshold and provide guidance to request a specific file for detail.
• Preserve both ends of the diff in the bounded fallback path if the summary command fails.
• Add utility and integration regression tests plus a patch changeset.

Validation

• pnpm run test:types
• pnpm run test:format
• pnpm run test:lint
• pnpm run test:ava source/tools/git/utils.spec.ts source/tools/git/git-diff.spec.tsx (36 passed)
• pnpm run test:all (all changed-area tests pass; one unrelated existing failure remains in source/app/components/settings-tabs.spec.tsx:109, where the test does not find the Display tab)

The full-suite failure is outside the files changed here and reproduces independently of this PR.

[RealBhupesh]

CI update: all required checks pass except Unit Tests & Coverage Analysis. The full npx c8 ava job reproduces the existing failure at source/app/components/settings-tabs.spec.tsx:109 (Display is not found in the rendered category tabs). This is outside the five files changed by the PR and matches the local pnpm run test:all result recorded in the PR description. CodeQL, build, type, format, lint, audit, Semgrep, unused-dependency, and changeset checks are green.