agent: P2-16 Windows service (SCM) integration

internal/agent/service: build-tagged into service_windows.go (svc.Handler
that listens for Stop/Shutdown + delegates to the agent loop) and
service_other.go (foreground stub for Linux/macOS). install_windows.go
wraps mgr.Connect+CreateService/Delete/Start/Stop for the new
'restic-manager-agent install|uninstall|start|stop' subcommands.

Cross-compile verified: GOOS=windows GOARCH=amd64 go build ./cmd/agent
succeeds. UNTESTED on Windows itself — the SCM round-trip can't be
exercised from Linux CI; treat as a starting point for the first
real Windows install.

internal/agent/service/install_windows.go

@@ -0,0 +1,103 @@
+//go:build windows
+
+// install_windows.go — thin wrappers around the Service Control
+// Manager via golang.org/x/sys/windows/svc/mgr. Used by the agent's
+// `install` / `uninstall` / `start` / `stop` subcommands.
+//
+// UNTESTED in CI. Mirrors the canonical example shape; if you need
+// to extend this, prefer copying from x/sys/windows/svc/example
+// over inventing new patterns.
+package service
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"golang.org/x/sys/windows/svc/mgr"
+)
+
+// Install registers the service with the SCM, pointing it at the
+// currently-running binary. The service starts on every boot and
+// runs as LocalSystem (default).
+func Install() error {
+	exe, err := os.Executable()
+	if err != nil {
+		return fmt.Errorf("install: locate executable: %w", err)
+	}
+	exe, err = filepath.Abs(exe)
+	if err != nil {
+		return fmt.Errorf("install: absolutise path: %w", err)
+	}
+	m, err := mgr.Connect()
+	if err != nil {
+		return fmt.Errorf("install: connect SCM: %w", err)
+	}
+	defer m.Disconnect()
+	if existing, err := m.OpenService(ServiceName); err == nil {
+		_ = existing.Close()
+		return fmt.Errorf("service %q already installed; uninstall first", ServiceName)
+	}
+	s, err := m.CreateService(ServiceName, exe, mgr.Config{
+		StartType:   mgr.StartAutomatic,
+		DisplayName: "Restic-manager agent",
+		Description: "Backs up this host on the schedule the central restic-manager dictates.",
+	}, "run")
+	if err != nil {
+		return fmt.Errorf("install: create service: %w", err)
+	}
+	defer s.Close()
+	return nil
+}
+
+// Uninstall removes the service from the SCM. Caller is expected to
+// stop the service first; this returns the SCM's error if it's
+// still running.
+func Uninstall() error {
+	m, err := mgr.Connect()
+	if err != nil {
+		return fmt.Errorf("uninstall: connect SCM: %w", err)
+	}
+	defer m.Disconnect()
+	s, err := m.OpenService(ServiceName)
+	if err != nil {
+		return fmt.Errorf("uninstall: open service: %w", err)
+	}
+	defer s.Close()
+	if err := s.Delete(); err != nil {
+		return fmt.Errorf("uninstall: delete service: %w", err)
+	}
+	return nil
+}
+
+// Start asks the SCM to start the installed service. No-op if it's
+// already running (the SCM returns an error which we surface).
+func Start() error {
+	m, err := mgr.Connect()
+	if err != nil {
+		return err
+	}
+	defer m.Disconnect()
+	s, err := m.OpenService(ServiceName)
+	if err != nil {
+		return err
+	}
+	defer s.Close()
+	return s.Start()
+}
+
+// Stop sends a stop control to the service.
+func Stop() error {
+	m, err := mgr.Connect()
+	if err != nil {
+		return err
+	}
+	defer m.Disconnect()
+	s, err := m.OpenService(ServiceName)
+	if err != nil {
+		return err
+	}
+	defer s.Close()
+	_, err = s.Control(0x00000001) // SERVICE_CONTROL_STOP
+	return err
+}