Replace the _fsForTest/DI patterns with Bun's native mock.module() API.
This mocks fs/promises at the module loader level, which works reliably
on Linux where jest.fn() call tracking fails cross-module. The source
code is now clean with no testing artifacts.
Also switches from jest.fn() to mock() (Bun's native API) for Octokit
mocks in these tests.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
The _fsForTest mutable object pattern fails on Linux Bun because
test and source files get different module instances. Switch to
passing fs operations as an optional parameter with defaults.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
