Phase 4: kids' Windows computer client (AUD-97)

Adds windows/, the Python client for the kids' Windows computers, and proves
both halves of the Phase 4 definition of done on the real box
(DESKTOP-TE2RHV2).

The client:
- subscribes, retained, to its own config on the 8883 TLS device plane with its
  own broker account (FR-3, FR-6);
- keeps the last valid config on disk and loads it before connecting, so a
  broker outage changes nothing (FR-14);
- terminates every process on the blocklist while the device is effectively
  locked, with the allowlist winning and the client's own tree plus critical
  system processes protected (FR-4, FR-16);
- publishes presence and daily per-app usage; the day's totals are held on disk
  and republished when the link returns (FR-7);
- shows an always-on-top warning window naming what is blocked, why and until
  when (FR-9).

install-scheduled-task.ps1 registers the logon Scheduled Task for the kid's
account with an interactive token, and grants that account Modify on the state
directory: a file created from an elevated shell inherits Administrators only,
and the limited task is otherwise refused with Access denied.

Evidence (windows/EVIDENCE.md, from the machine's own log): notepad.exe killed
inside a scheduled window (pid 16600, then 9724), the device online in the
Control Service with locked/until/reason, a telemetry row for the device
(app=690s), enforcement continuing with the broker unreachable
(192.168.20.13:8884), the warning window on the interactive desktop, and the
Scheduled Task registered at logon as New Comp.

Domains are deliberately not reported: Windows has no cheap per-process DNS
attribution, so the client sends "domains": [] and the gap is documented rather
than guessed.

54 unit tests, run with `make windows-test` (no broker, no Windows, no psutil).

AGENTS.md §11 is now 10 of 10.

AGENTS.md

@@ -189,14 +189,15 @@ The Hub interface is frozen in `central_ai_llm_master/docs/ask-contract.md`.
 
 ## 11. Build order
 
-**STATUS — 9 of 10 done — only Phase 4 outstanding (checked 2026-10-14).** Keep
+**STATUS — 10 of 10 done — Phase 4 passed its DoD on the real Windows box
+(checked 2026-10-14).** Keep
 this list current: tick a box when its DoD passes. **The project is DONE only when
 every box is ticked.**
 
 - [x] Phase 1 — Skeleton
 - [x] Phase 2 — Devices and state
 - [x] Phase 3 — MQTT
-- [ ] **Phase 4 — Computer client (Windows)**  ← REMAINING
+- [x] Phase 4 — Computer client (Windows)
 - [x] Phase 5 — Phone client (no-wipe)
 - [x] Phase 6 — Schedules, blocklists, allowlists
 - [x] Phase 7 — Telemetry and usage