feat(windows): platform ops, service wrapper, helper, and air-gap paths

First Windows parity milestone (DESIGN-WINDOWS.md §13 build order item 1).

- Add a PlatformOps abstraction so command dispatch is OS-neutral:
  - linuxPlatformOps keeps today's systemctl/journalctl/bash behavior (deliberately
    untagged so shared dispatch tests run on Windows CI)
  - windowsPlatformOps maps reboot/shutdown to shutdown.exe, service control to
    sc.exe (stop+start for restart), fetch_logs to Get-WinEvent, arbitrary_bash to
    powershell -EncodedCommand (byte-exact under arbitrary quoting), and declines
    configure_ldap (Windows logon goes through OpenCredential)
- Run theta-agent as a Windows service (x/sys/windows/svc): SYSTEM auto-start,
    SCM stop/shutdown handling; CLI install-service/remove-service via svc/mgr
- Add theta-agent-helper (session-0 companion): lock/display_off/logout via
    user32/wtsapi32, and staged self-update (wait for service stop, swap the
    locked exe, sc start)
- Self-update becomes platform-aware: Linux renames over the running binary;
    Windows stages .new and hands the swap to the helper (running exe is locked)
- Platform paths: agent.yml and tray.sock under %ProgramData%\Theta42 (the
    service runs as SYSTEM while the tray runs as the user, so the per-user temp
    dir no longer works for tray IPC); LDAP byte-pump falls back to TCP loopback
- config: service_name, desktop_helper, public_ip_detect (air-gap: skips
    external public-IP lookups in telemetry + home monitor), wireguard block
- cli: platform-aware config path + self-update artifact name + service restart
- tests: dispatch tests pin linuxPlatformOps; 0600 mode assertions gated to
    POSIX so the suite is green on Windows

Rebuilds all tracked dist binaries (v2.1.0).

service_windows.go

@@ -0,0 +1,65 @@
+//go:build windows
+
+package main
+
+// Windows service wrapper. The installer registers theta-agent as a SYSTEM
+// auto-start service; when the SCM launches the process this file takes over.
+// The service is up before any user session, which is what allows the
+// credential provider to validate LDAP logins at Ctrl+Alt+Del and what makes
+// the tray IPC socket path (shared data dir) correct.
+
+import (
+	"log"
+	"os"
+
+	"golang.org/x/sys/windows/svc"
+)
+
+// maybeRunAsService returns true (and runs the service loop) when launched by
+// the Service Control Manager; false when run in a console.
+func maybeRunAsService() bool {
+	isSvc, err := svc.IsWindowsService()
+	if err != nil || !isSvc {
+		return false
+	}
+	if err := svc.Run("theta-agent", &agentService{}); err != nil {
+		log.Printf("service: %v", err)
+		os.Exit(1)
+	}
+	return true
+}
+
+type agentService struct{}
+
+func (s *agentService) Execute(args []string, r <-chan svc.ChangeRequest, changes chan<- svc.Status) (bool, uint32) {
+	const cmdsAccepted = svc.AcceptStop | svc.AcceptShutdown
+
+	changes <- svc.Status{State: svc.StartPending}
+
+	done := make(chan struct{})
+	go func() {
+		defer close(done)
+		runAgent()
+	}()
+
+	changes <- svc.Status{State: svc.Running, Accepts: cmdsAccepted}
+
+	for {
+		select {
+		case c := <-r:
+			switch c.Cmd {
+			case svc.Interrogate:
+				changes <- c.CurrentStatus
+			case svc.Stop, svc.Shutdown:
+				changes <- svc.Status{State: svc.StopPending}
+				stopAgent()
+				<-done
+				changes <- svc.Status{State: svc.Stopped}
+				return false, 0
+			}
+		case <-done:
+			changes <- svc.Status{State: svc.Stopped}
+			return false, 0
+		}
+	}
+}