Expand Windows discovery and browser-launch coverage

[NoureddinBakir]

Problem

Filesystem safety and process launching differ across macOS, Linux, and Windows. The public CI suite currently runs only on Linux.

Proposed direction

Add Windows CI and focused tests for path normalization, home-directory discovery, mode limitations, and the explicit browser-launch flow.

Acceptance criteria

• Adds a Windows runner to CI without weakening existing Linux coverage.
• Tests drive-letter and separator normalization without leaking absolute paths.
• Verifies that symlink or reparse-point handling remains conservative.
• Covers the cmd /c start argument shape without launching a real browser in tests.
• Documents any filesystem permission guarantees that cannot be replicated on Windows.
• Keeps uploads pinned to the existing production origin.