Make /bug open GitHub correctly on Windows

[amoreX]

/bug uses open on macOS and xdg-open everywhere else. Native Windows therefore attempts to execute xdg-open.

Acceptance criteria

• macOS uses open.
• Linux uses xdg-open.
• Windows uses start, PowerShell, or another safe native mechanism.
• Arguments must not be vulnerable to shell injection.
• If opening fails, keep displaying the issue URL.
• Add platform-selection tests.