feat(daemon): Windows lid auto-pause via SystemEvents.PowerModeChanged

[scokeepa]

Summary

• Subscribes cmux-daemon to SystemEvents.PowerModeChanged and toggles %TEMP%\cmux-paused.flag on Suspend/Resume.
• New file src/Cmux.Daemon/LidPauseWatcher.cs; one-line hookup in Program.cs; Microsoft.Win32.SystemEvents 9.0.0 added to Cmux.Daemon.csproj.
• The flag file is the cross-platform SSOT shared with cmuxO PR #5 (macOS sleepwatcher via ~/.sleep / ~/.wakeup). Orchestration skills poll this single file on both platforms.

Why here (daemon, not WPF app)

cmux-daemon is the always-on background process. Hooking there means lid events are captured even when no WPF window is open, and the logic stays out of UI code.

Why SystemEvents.PowerModeChanged

• Built-in managed message pump — no hidden NativeWindow, no WM_POWERBROADCAST P/Invoke.
• Fires for sleep/hibernate/resume regardless of lid vs Start-menu sleep.
• Package Microsoft.Win32.SystemEvents is the supported path for non-WinForms/WPF executables on net*-windows.

Alternative considered: PowerRegisterSuspendResumeNotification (powrprof.dll) — rejected for complexity; no advantage for this use case.

Test plan

• dotnet build Cmux.sln -c Debug succeeds on Windows (unverified — authored on macOS, no local .NET 10 Windows SDK).
• Start cmux-daemon, close laptop lid / trigger Sleep → %TEMP%\cmux-paused.flag exists; daemon-debug.log shows [lid-watcher] Suspend.
• Resume → flag removed; log shows [lid-watcher] Resume.
• Hibernate/Resume cycle → same behavior.
• Kill daemon with flag present → orphaned flag is cleaned by next Resume (acceptable: worst case a stale flag until next suspend/resume).

Risks / follow-up

• Unverified build: authored on macOS; reviewer please confirm net10.0-windows resolves Microsoft.Win32.SystemEvents 9.0.0 cleanly (may want to pin to 10.0.0 once published to match TFM).
• Stale flag on crash: if the daemon crashes between Suspend and Resume, the flag persists. Could add a startup cleanup (File.Delete(FlagPath) before LidPauseWatcher.Start) if this proves annoying in practice — left out of this PR to keep it minimal.
• Single-instance mutex: existing Global\CmuxDaemon mutex ensures only one subscriber, so no duplicate flag toggling.

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