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).

config_test.go

@@ -3,6 +3,7 @@ package main
 import (
 	"os"
 	"path/filepath"
+	"runtime"
 	"strings"
 	"testing"
 )
@@ -155,10 +156,13 @@ capabilities:
 		t.Errorf("Credential() should prefer the issued token, got %q", cm.Get().Credential())
 	}
 
-	// file must stay 0600 -- it now holds a credential
-	fi, _ := os.Stat(path)
-	if fi.Mode().Perm() != 0600 {
-		t.Errorf("expected mode 0600, got %o", fi.Mode().Perm())
+	// file must stay 0600 -- it now holds a credential. POSIX-only: Windows has
+	// no mode bits (0666 is reported regardless) and relies on ACLs instead.
+	if runtime.GOOS != "windows" {
+		fi, _ := os.Stat(path)
+		if fi.Mode().Perm() != 0600 {
+			t.Errorf("expected mode 0600, got %o", fi.Mode().Perm())
+		}
 	}
 }