feat(core-testing): git-serving fixture helpers

serveFixtureRepo(fixtureDir) copies a fixture into a temp dir, commits
it as a fresh single-commit repo, bare-clones it, and serves it over
`git daemon --export-all` on an ephemeral localhost port (default) or
as a file:// URL (fallback for daemon-less environments). Returns
{ cloneUrl, bareRepoPath, protocol, stop } with idempotent teardown.
Tests exercise a real `git clone` of fixtures/vite-kitchen over both
protocols. New subpath export @repo/core-testing/git — node-only, so
deliberately not on the jsdom root barrel. No new runtime deps: node
built-ins + system git (present in git's exec-path on macOS and the
ubuntu CI image).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016j8z4VHjedXDTjEDNg7qHK

packages/core-testing/AGENTS.md

@@ -6,6 +6,7 @@ Shared testing utilities. Tag: `tooling`. May be depended on by any package as a
 
 - `@repo/core-testing/factory` — `defineFactory<T>(builder)` for test data factories
 - `@repo/core-testing/contract` — `defineContractSuite<T>(name, suite)` for cross-impl contract tests
+- `@repo/core-testing/git` — `serveFixtureRepo(fixtureDir, { protocol? })` serves a fixture directory (e.g. `fixtures/vite-kitchen`) as a real git remote for integration tests: copies it to a temp dir, commits it, bare-clones it, and exposes it via `git daemon` on an ephemeral localhost port (default) or a `file://` URL (fallback). Returns `{ cloneUrl, bareRepoPath, protocol, stop }`; always `await stop()` in `afterEach`. Node-only (`node:child_process` + system git) — intentionally not re-exported from the root barrel
 - `@repo/core-testing/react` — `renderWithProviders`, `createMockTrpcClient`
   - `renderWithProviders` does NOT include a tRPC provider. Consumers needing tRPC should wire their own TRPCProvider (from their app's tRPC client setup) and use `createMockTrpcClient` as the client. This constraint exists because tooling packages cannot import `AppRouter` from `@repo/core-api`.
 - `@repo/core-testing/payload` — `stubPayloadConfig`, `mockPayloadModule`