Intermittent Windows CI timeouts in tests that clone from GitHub

Some unit tests (`ConfigLoader.test.ts`, `clearBareClone.test.ts`) perform real git clone against GitHub. This causes intermittent timeout failures on Windows CI (https://github.com/finos/git-proxy/actions/runs/26777705958/job/78933593470), where network operations are significantly slower than on Ubuntu runners.

Proposed solution

Replace GitHub clones in unit tests with local `file://` repositories. This requires to add `file://` support to isValidGitUrl, then we could move network-dependent tests to e2e test

Start with ConfigLoader.test.ts and clearBareClone.test.ts to identify the GitHub clone cases, then inspect isValidGitUrl and the existing end-to-end tests. Replace unit-test network clones with local file:// repositories, add file:// URL support, and move network-dependent cases to e2e tests so Windows CI no longer relies on slow GitHub operations.

Written by the indexing model from the issue text.