daemon: split owner daemon from root helper

Move the supported systemd path to two services: an owner-user bangerd for
orchestration and a narrow root helper for bridge/tap, NAT/resolver, dm/loop,
and Firecracker ownership. This removes repeated sudo from daily vm and image
flows without leaving the general daemon running as root.

Add install metadata, system install/status/restart/uninstall commands, and a
system-owned runtime layout. Keep user SSH/config material in the owner home,
lock file_sync to the owner home, and move daemon known_hosts handling out of
the old root-owned control path.

Route privileged lifecycle steps through typed privilegedOps calls, harden the
two systemd units, and rewrite smoke plus docs around the supported service
model.

Verified with make build, make test, make lint, and make smoke on the
supported systemd host path.

internal/daemon/doctor.go

@@ -16,14 +16,15 @@ import (
 )
 
 func Doctor(ctx context.Context) (system.Report, error) {
-	layout, err := paths.Resolve()
+	userLayout, err := paths.Resolve()
 	if err != nil {
 		return system.Report{}, err
 	}
-	cfg, err := config.Load(layout)
+	cfg, err := config.Load(userLayout)
 	if err != nil {
 		return system.Report{}, err
 	}
+	layout := paths.ResolveSystem()
 	// Doctor must be read-only: running it should never mutate the
 	// state DB (no migrations, no WAL checkpoint, no pragma writes).
 	// Skip OpenReadOnly entirely when the DB file doesn't exist —
@@ -32,9 +33,10 @@ func Doctor(ctx context.Context) (system.Report, error) {
 	// "no DB yet" (pass) from "DB present but unreadable" (fail) in
 	// the report.
 	d := &Daemon{
-		layout: layout,
-		config: cfg,
-		runner: system.NewRunner(),
+		layout:     layout,
+		userLayout: userLayout,
+		config:     cfg,
+		runner:     system.NewRunner(),
 	}
 	var storeErr error
 	storeMissing := false
@@ -90,7 +92,7 @@ func (d *Daemon) doctorReport(ctx context.Context, storeErr error, storeMissing
 // This is intentionally a warn, not a fail — the shortcut is opt-in
 // convenience and `banger vm ssh` works either way.
 func (d *Daemon) addSSHShortcutCheck(report *system.Report) {
-	bangerConfig := BangerSSHConfigPath(d.layout)
+	bangerConfig := BangerSSHConfigPath(d.userLayout)
 	if strings.TrimSpace(bangerConfig) == "" {
 		return
 	}