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).

go.sum

@@ -37,8 +37,6 @@ golang.org/x/sys v0.8.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.11.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.20.0 h1:Od9JTbYCk261bKm4M/mw7AklTlFYIa0bIp9BgSm1S8Y=
 golang.org/x/sys v0.20.0/go.mod h1:/VUhepiaJMQUp4+oa/7Zr1D23ma6VTLIYjOOTFZPUcA=
-golang.org/x/sys v0.47.0 h1:o7XGOvZQCADBQQ4Y7VNq2dRWQR7JmOUW8Kxx4ZsNgWs=
-golang.org/x/sys v0.47.0/go.mod h1:4GL1E5IUh+htKOUEOaiffhrAeqysfVGipDYzABqnCmw=
 golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=