DIFF-475：Vault Git 清空 remote 与 Electron 对齐

背景

Electron VaultGitService.configureRemote 在保存空 URL 时执行 git remote remove origin；status().remote 变为空。Compose GitEngine.setRemote 已有相同逻辑，但缺少与 Vitest 用例「removes a configured remote when an empty URL is saved」及「Remote is already removed」的显式单测。

变更

• GitEngineTest.configureRemoteRemoveClearsOriginFromStatus
• GitEngineTest.configureRemoteRemoveWhenAlreadyRemovedIsIdempotent
• docs/fixtures/electron-next-vaultGitService-vitest.md 登记 configure-remote-remove

验证

export JAVA_HOME="$(/usr/libexec/java_home -v 21)"
./gradlew :composeApp:desktopTest --offline