feat: add psmux session backend for windows

[matej21]

Summary

• Adds psmux as a Windows session backend, mirroring Unix dtach/tmux/screen integration. When psmux is on PATH, okena auto-detects it and wraps non-WSL shells through psmux new-session -A, so PowerShell/cmd sessions survive okena restart.
• WSL terminals continue to use their own per-distro backend inside the distro (psmux is host-Windows only). Selecting Psmux for a WSL terminal falls back to dtach/tmux/screen inside WSL.
• Same UX as Unix: install psmux (winget install psmux / cargo install psmux / scoop install psmux), okena handles the rest. No manual attach.

Implementation notes

• SessionBackend::Psmux + ResolvedBackend::Psmux variants; Auto resolution prefers psmux on Windows, dtach > tmux > screen on Unix (unchanged).
• is_psmux_available() runs psmux -V via crate::process::command (CREATE_NO_WINDOW).
• build_command for Psmux: bypasses host shell (cmd.exe doesn't honor sh-style quoting); ; is psmux's literal command separator when received as its own argv token.
• kill_session: psmux kill-session -t <name>.
• Windows pty_manager::build_terminal_command now wraps non-WSL shells via host backend through a wrap_with_host_backend closure.

Known limitations (documented in code)

• okena's default_shell setting isn't propagated into psmux sessions — psmux uses its own default-shell from ~/.psmux.conf (PowerShell out of the box). Custom command override via cmd.exe /c works.

Test plan

• All 80 okena-terminal tests pass on Linux host
• All 163 okena-workspace tests pass
• okena-terminal cross-checks cleanly for x86_64-pc-windows-gnu
• Added 5 new tests: build_command minimal/custom/extra_env, socket_path returns None, supports_persistence
• Verified non-exhaustive matches in WSL paths (resolve_for_wsl, build_wsl_session_command, kill_wsl_session) handle the new variant correctly
• Manual test on Windows: install psmux, launch okena, open PowerShell terminal, verify session persists across okena restart
• Manual test on Windows: WSL terminal continues to use per-distro backend (no regression)

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