Installer registers FalloutAgent as a Windows Service

Parent: #121

The Windows installer (from __W2__) registers `FalloutAgent` as a Windows Service so it survives reboots and is manageable via `services.msc` / `sc.exe`.

## Definition of done

- Service name: `FalloutAgent` (display name: `Fallout Deployment Agent`).
- Runs under `NT AUTHORITY\LocalService` by default (or `NetworkService` if coordinator connectivity requires it — decide during implementation).
- Start type: `Manual` initially; operator opts into `Automatic` after first successful enrolment. (Rationale: avoid a fresh install spam-failing because no config exists yet.)
- Recovery actions: restart on failure, with backoff.
- Uninstall stops + removes the service cleanly; no orphaned service entries.
- Re-install over an existing install upgrades without data loss in `%ProgramData%\Fallout\Agent\`.