test: migrate simple-git from jest to vitest

[steveukx]

Stacked on #1170 (pnpm). Replaces jest + babel-jest with vitest for the simple-git package (plan PR 2). Targets v4/pnpm-migration so it reviews as a clean diff on top of the pnpm work; both land on canary/v4.

Test infrastructure

• vitest.config.mts with two projects:
  • unit — mocks child_process (+ debug, @kwsites/file-exists) via a setup file.
  • integration — no child_process mock, real git, longer timeouts.
    This mirrors the old jest split where only unit specs imported the manual child_process mock; a single global mock would have hung the real-git integration tests.
• The three jest.mock() calls move into test/unit/__fixtures__/setup.ts as vi.mock(), with the debug factory inlined to satisfy vi.mock hoisting (the exact trap called out in the plan).
• mock-child-process.ts + fixtures: jest.* → vi.*; jest.Mock type → Mock from vitest across all specs.
• Aliases for simple-git / simple-git/promise / typings to source; @simple-git/test-utils inlined so its import 'simple-git' resolves to TS source rather than being externalised.

Source entry points (the necessary prerequisite)

vitest can't execute the legacy CommonJS entries that require() sibling .ts files extensionlessly — Node's CJS resolver runs those requires and can't find .ts. So:

• src/index.js → src/index.ts (keeps export = for the callable require('simple-git') CJS shape).
• src/git.js → src/git.ts, body kept verbatim under // @ts-nocheck (it's the 600-line prototype file the task-refactor PR rewrites; type-checking it now is out of scope).
• Dev main → src/index.ts (publishConfig still overrides to dist for publishing).

Package

• test → vitest run --coverage; test:win → vitest run win32.
• Drop jest, @types/jest, ts-node, @simple-git/babel-config; delete jest.config.js / babel.config.js; add vitest + @vitest/coverage-v8.
• test-typescript-consumer still uses jest (separate migration; full babel removal is its own PR).

§0.1 acceptance

All 787 tests pass (exactly matching the previous jest count) at 94.45% coverage (≥80 gate). build, build:pkg, and all consumer tests (ESM, CJS callable+named+default, TS types) remain green.

Note for review

git.spec.ts's CJS-interop test (__esModule / default === self via require('../..')) now asserts the equivalent source-level guarantees (typeof default === 'function', simpleGit === default). The built-CJS __esModule shape is still covered by the JS consumer tests. Flagging since it's the one place an assertion changed rather than just being ported.

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