spec: add SPEC-018 managed-agent SYSTEM service host + session broker

LocalSystem service that runs the persistent agent unattended and brokers
per-session capture/input workers (Session 0 can't capture directly).
Unblocks SPEC-016 Phase B end-to-end (SYSTEM-ACL'd cak_ store readable;
removes the Phase B fail-fast guard) and is the broker primitive SPEC-013
builds on. 017 was taken by Mike's end-user-access spec, so this is 018.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/FEATURE_ROADMAP.md

@@ -68,6 +68,7 @@ Bringing GC to parity with GuruRMM's release engineering. Full plan: [SPEC-001](
 - [x] Protobuf-over-WSS transport, Zstd frame compression
 - [~] React/TS web viewer (`dashboard/src/components/RemoteViewer.tsx`) — embeddable session viewer
 - [ ] **Headless Linux mode (direct TTY access)** — P2 — Terminal-based remote access for Linux servers without GUI. PTY spawn (`openpty`), xterm.js web viewer, full ANSI/VT100 support. Enables server management, container debugging, emergency recovery via GuruConnect dashboard with audit logging. SSH replacement with centralized auth. ([SPEC-012](specs/SPEC-012-headless-linux-tty.md))
+- [ ] **Managed-agent SYSTEM service host + session broker** — P1 — convert the persistent agent from `HKCU Run` (user context) to a LocalSystem **service** that runs unattended (login screen, no user, across reboots) and spawns a per-session capture/input worker into the active desktop (Session 0 can't capture directly). Unblocks SPEC-016 Phase B end-to-end (the SYSTEM-ACL'd `cak_` store becomes readable; removes the Phase B fail-fast guard), enables true unattended access, and is the **broker primitive SPEC-013 builds on**. ([SPEC-018](specs/SPEC-018-managed-agent-service-host.md))
 - [ ] **Windows session selection and backstage mode** — P2 — Enumerate and switch between Windows user sessions (Terminal Services/RDP/Fast User Switching) and access Session 0 (backstage) for system-level admin tasks. ScreenConnect parity: session selector shows all logged-on users, instant switching without reconnect. Backstage mode provides terminal/command interface for services management without disrupting any user desktop. Critical for multi-user server environments. ([SPEC-013](specs/SPEC-013-session-selection-and-backstage.md))
 - [ ] **Configurable notification overlay on viewer connection** — P2 — Display a semi-transparent on-screen notification when a technician connects, showing technician name and company. Dashboard-configurable message template (supports `{{technician_name}}`, `{{company}}`, `{{time}}`), duration (5-60s), position (top-left/right, bottom-left/right, center), and dismissible behavior. Increases transparency and user awareness during remote support sessions. Compliance-friendly for privacy policies requiring user notification. ([SPEC-015](specs/SPEC-015-notification-overlay.md))
 - [ ] Multi-monitor switching — P2