Run and document Windows remote manual E2E before v1.0

[devgamesan]

Context

The codebase and manual_tests include Windows remote support, but the latest E2E confirmation covered only the Linux target g3plus. If Windows is part of the v1.0 support promise, the Windows manual E2E flow should be exercised and documented.

Tasks

• Run uv run pytest manual_tests -vv with GSS_TEST_WINDOWS_HOST and GSS_TEST_WINDOWS_USER.
• Record the exact environment and command used.
• Fix any Windows-specific failures in path quoting, SSH command execution, Git remote URLs, or cleanup.
• If Windows cannot be validated for v1.0, mark Windows support as experimental in README/README.ja.

Acceptance Criteria

• Either Windows manual E2E passes and is documented as a v1.0 validation target, or Windows support is explicitly documented as experimental.
• Cleanup leaves no manual test branches, tags, work repos, or cache repos behind.
• docs/manual-testing.md reflects the validated Windows command.